[page 14↓]

2  Review of literature

2.1 Production performance of sheep with relevance to the tropics

2.1.1 Performance of mutton sheep breeds and their crosses under tropical conditions

2.1.1.1 Growth performance

It is generally accepted that crossbreeding local small ruminants in the Tropics with exotic breeds could contribute to increased animal productivity. MOHAN et al.(1985) recorded very significantly (p < 0.01) higher ADG in Nellore x Dorset crosses than in Mandya x Dorset ones. FERNANDES and DESHMUKH (1986) found significantly (p < 0.05) higher live body weight at weaning (90d) and at 180 days of age of Dorset x Deccani crosses than of the pure Deccani, Merino x Deccani or the C2 of both C1 genotypes. Feed efficiency was also significantly (p < 0.05) higher for the crosses with Dorset followed by those with Merino.

Among tropical breeds of sheep hair sheep are considered to have high potential to increase productivity under tropical conditions where animal protein is in short supply. Hair sheep (BRADFORD BRADFORD and FITZHUGH, 1983; PASTRANA et al., 1983; ZARAZUA and PADILLA, 1983) and crosses between them and temperate breeds (GOODE et al., 1983) are well adapted to the tropical environments. GOODE et al.(ibid.) recorded a rectal temperature in Barbados Blackbelly ewes of 0.7°C below that of the Dorset and Suffolk ones whereas that of their crosses was intermediate. The range of adaptation of hair sheep also includes tolerance to trypanosomiasis (ADEMOSUN et al., 1983; BERGER, 1983) and worm infestation (GOODE et al., ibid.).

There is immediate need to increase the productive performance of small ruminants in the Tropics which is very low at the moment. ARMBRUSTER and PETERS (1992) reported low adult live weights for Djallonke sheep and goats aged twenty-four (24) months of 21.8 kg and 18.1 kg, respectively. Average litter size was 1.19 lambs and 1.52 kids, respectively. Reproductive performance was however high with annual lamming and kidding rates of 173.6% and 234.1%, respectively. The trend was reported to be similar in West Africa for both types of small ruminants (ibid.).

Biological and economic efficiency of lamb production can be achieved first and foremost through increased reproductive rate i. e. by increasing the number and weight of the offspring of the breeding female (METZ, 1990). Biological improvements need to be accompanied by better management. KEZIE (1997) reported a 38% improvement in gross margin per ewe, of sheep reared under relatively improved management in Togo as being attributed to higher reproductive performance.

The higher performance of crosses of hair sheep with exotic breeds compared with pure hair sheep and expected adaptation could be further exploited to increase supply of animal protein in the Tropics. Crossing West African Dwarf with Blackhead Persian increased birth weight (2.3 ±0.45 kg) of the crosses more than that of the locals (1.3 ±0.23 kg) and pushed the adult weight of the ewes to 32 kg instead of 21 kg in the local and still within limits to survive under natural grazing conditions (NGERE, 1973). NGERE and ABOAGYE (1981) also found that the crossbred Nungua Black Head ewes produced very significantly (p < 0.01) heavier lambs that weighed 2.2 kg at birth than the pure West African Dwarf sheep (1.3 kg) although comparatively, the former had more single births than twins.

GATENBY et al. (1997) found that crosses of St. Croix and Barbados Blackbelly with Sumatra gave more productive C1 ewes which produced bigger lambs than the local Sumatra. Crosses with Barbados Blackbelly (2.10 ±0.05 kg) and with St. Croix (2.03 ±0.03 kg) had highly significant (p < 0.001) birth weight than the Sumatra (1.45 ±0.04 kg).

FOOTE et al.(1983.) showed that crosses between St. Croix and Rambouillet achieved higher ADG than those of the pure St. Croix over a six week feeding period. However, the ADG of the pure Rambouillet was highest over the same period. GOODE et al.(ibid.) recorded significantly heavier birth weight of Dorset x Barbados Blackbelly crosses than those of Dorset, Dorset x Finnish Landrace and Rambouillet x Finnish Landrace crosses. Heavier birth weight was later followed by a higher ADG value. In another experiment, SHELTON (1983) recorded lowest ADG in Barbados Blackbelly x Rambouillet crosses compared with those of the Rambouillet and the Blackface x Rambouillet crosses. Nevertheless, Barbados Blackbelly x Rambouillet crossbred ewes had the biggest number of lambs weaned per ewe per year compared with the pure Rambouillet, Finnish Landrace x Rambouillet and the Karakul x Rambouillet. NURSE et al. (1983) have recorded highest litter size in the Barbados [page 15↓]Blackbelly ewes compared with the Creole and crosses between them. High litter size was, however, associated with high mortality (ibid.; DETTMERS, 1983) with about 66% of deaths taking place during the first week of lactation in the Barbados Blackbelly (ZARAZUA and PADILLA, 1983). Survival and growth are largely dependent on the milk yield of the ewes and is closely associated with birth weight, birth type and management (DETTMERS, ibid.). In some cases, survival and growth rate could be improved by crossbreeding (NURSE et al., ibid.). For tropical conditions, the issue of survival needs to be given due attention.

The trend of the crosses to perform better than local animals has also been observed in goats as well under tropical conditions. Comparing the performance of Galla goats and their crosses with Anglo-Nubian, ABEBE (1996) also recorded higher growth rates for crosses at weaning than for the pure Galla both under conditions of natural pasture and/or occasional supplementation.

Table 1 shows the performance of hair sheep with regard to birth weight and Average Daily Gain. Comprehensive studies looking at both pre- and post-weaning performance of hair sheep and their crosses which also consider milk performance as an essential trait for lamb survival have been wanting in the tropics.

It is not clear if the matted coat of the C1 crosses produced by crossing hair sheep with wool breeds would allow for adequate adaptation to a tropical environment (BRADFORD BRADFORD and FRITZHUGH, 1983). On account of their new coat and vlies types, crossbred lambs may have a physiologically different pattern of reaction to high ambient temperature than pure hair sheep which may have serious implications for production.


[page 16↓]

Table 1: Birth weight and Average Daily Gain performance of hair sheep and their crosses

Breed/Cross

Birth wt., kg

ADG, g

Stage

Management

Zone/Country

References

West African Dwarf (WAD)

1.50 - 1.70

-

-

Free range

Humid Tropics

ADEMOSUN et al., 1983

WAD

2.40

-

-

Station

’’

ADEMOSUN et al., 1983

WAD

1.60

-

-

’’

’’

DETTMERS, 1983

WAD

1.77

-

-

’’

’’

DETTMERS, 1983

WAD x PERMER

2.30

-

-

’’

’’

DETTMERS, 1983

WAD x PERMER

1.48 - 1.86

-

-

 

’’

ADEMOSUN et al., 1983

“Small Forest Sheep”

-

57 – 85

30 - 150d

’’

’’

VALLERAND/BRANCKAERT, 1975 quoted by DETTMERS, 1983

Djallonké

-

51 – 87

30 - 120d

’’

’’

BERGER, 1983

Djallonké

1.70

79

7 - 30d

’’

’’

BRADFORD, 1983

  

44

30 - 90d

   

Pelibuey

2.50

-

-

’’

’’

ZARAZUA and PADILLA, 1983

Peliguey

2.50

-

-

’’

Pasture

GONZALEZ-REYNA et al., 1983

Peul

-

124

0 - 40d

’’

Arid Tropics

BRADFORD, 1983

  

111

40 - 180d

   

Touabire

-

146

0 - 40d

’’

Arid Tropics

BRADFORD, 1983

  

93

40 - 180d

   

“Sahel”

3.0

84

0 - 180d

Sedentary

Arid Tropics

WILSON, 1983

Yankasa

3.75

-

-

-

’’

FERGUSON, 1964, quoted by DETTMERS, 1983

Uda

3.75

-

-

-

’’

’’

Merino x Yankasa

3.80

-

-

-

’’

’’

Merino x Uda

4.20

-

-

-

’’

’’

Dorset x Barbados Blackbelly (BB)

3.4 0 - 3.90

  

Station

USA

GOODE et al., 1983

BB x Dorset

-

200

post-weaning

’’

’’

GOODE et al., 1983

Dorset x (Dorset x BB)

-

290

’’

’’

’’

GOODE et al., 1983

Suffolk x (Dorset x BB)

-

290

’’

’’

’’

GOODE et al., 1983

St. Croix

-

255

’’

’’

’’

FOOTE, 1983

St. Croix x Rambouillet

-

292

’’

’’

’’

FOOTE, 1983


[page 17↓]

2.1.1.2  Importance of vlies

The range of temperature conditions under which sheep are kept for the production of meat and wool is extremely wide, varying from subtropical to subarctic conditions (GRAHAM et al., 1959). This may constitute an advantage with regard to any effort made to increase productivity by crossbreeding of various relevant genotypes and more especially those involving hair sheep. Nevertheless, the extent to which coat and vlies type of each new cross influences physiological reaction and therefore productivity needs due consideration. BLIGH (1959) came to the conclusion that panting in response to high ambient temperature was peripheral in origin thus highlighting the important role of peripheral thermoreceptors in thermoregulation.

Hair colour and density along with live body weight are some of the visible physical adaptations to environmental conditions (JOHNSON, H and RAGSDALE, 1960). PANT et al.(1985) found that the colour (black or white) of hairy sheep and goats under tropical conditions in Brazil did not influence mean rectal temperature; however, sheep had 8.55 more (p < 0.001) respirations per minute than goats. From the morning time to the afternoon, respiration rate increased (p < 0.0001) almost three times. Mean respiration rate for black animals was higher (43.31) than for white animals (38.30), a fact caused by large differences within goats (black and white) than within sheep.

Vlies is an insulation agent whose effectiveness is based on its ability to form a relatively stagnant layer of air around it which tends to block wind and thereby blocking the convection of warmth (BIANCA, 1971). Unlike solar radiation, wind lowers the level of the lower critical termperature (BIANCA, 1968). The most stressful combination of factors affecting this is that of low temperature, high wind and rain (BIANCA, 1976). The insulatory ability of vlies can be adversely affected (reduced) by strong wind and by water whose conductivity is twenty-four (24) times that of air (BIANCA, 1971).

Through standing vlies and change over from summer to winter vlies, the insulatory capacity of vlies is increased short-term and long-term, respectively. Seasonal changes in vlies characteristics are incited by a reduction in the photoperiod and by the lowering down of temperature which is more of a feature of the temperate regions than of the tropics (ibid.).

The growth of vlies varies between animals depending on the level of selection. Double coats with differential physical characteristics (i. e short finer undercoat and long coarse overcoat) typical of the Angora rabbit, cashmere goats, mink and wild sheep exhibit a seasonal pattern of moulting. Seasonal moult of vlies is a means to renew and modify its structure in order to adapt to seasonal climatic changes. Involved in the neuro-endocrine control of the growth and moulting of vlies are melatonin and prolactin, hormones secreted by the pineal body and the pituitary gland, respectively (ALLAIN, 1993; ALLAIN et al., 1994).

A smooth hair coat with short, thick medullated hair is very effective in the reflection of solar radiation (BIANCA, 1976).

Under conditions of solar radiation, WALI and ASHIR (1990) recorded a significant rise in both skin and wool temperatures of Awassi sheep than in the shade that suggested that thermoregulation took place more through sweating than through panting.

2.1.1.3 Milk yield performance

Data on milk yield performance of hair sheep and their crosses is lacking. It is expected that sheep will exhibit a milk yield performance similar to that of goats such that their pattern of reaction can reasonably be compared.

Milk yield of the ewe is the main factor affecting lamb growth during the first few weeks of life (COOMBE et al., 1960). Ewe milk yield and growth of the lamb are highly correlated during the 4 - 6 weeks of age (OWEN, 1957). OWEN (ibid.) also discussed a number of factors that influence the milk yield of the ewes that include level of nutrition, body weight of the ewe, age of the ewe, birth weight of the lambs, udder development and genotype of the ewe. Adequate good quality nutrition is essential during the late stages of gestation and during lactation. Milk yield tends to be positively correlated with body weight and also tends to increase in successive lactations as the ewe gets older. And under similar conditions of rearing and management milk yield of different breeds of sheep may be different. COOMBE et al.(ibid.) associated the later stages of lactation with deterioration in the temperament of the ewes to stand long enough to allow the lambs enough opportunity to suckle under field conditions.


[page 18↓]

METZ (1990) foresaw the high potential of synthetic breeds of tropical and temperate zone goats involving the Malaysian Katjang and German Fawn, respectively, in terms of increased mature live body weight, high growth rate and reduced antagonism between fecundity and rearing ability. It was recognised that the improvement of maternal performance through crossbreeding reduced this form of antagonism and was therefore especially important in small ruminants due to great variation in litter size and the fact that slaughter weight was largely attributed to maternal ability (BRADFORD, 1972).

There is therefore high potential to improve maternal performance and productivity through crossbreeding of local and temperate sheep breeds selected for high productivity. High milk yield of the ewe which is a pre-requisite for lamb survival and growth is the most important maternal improvement where lamb survival is especially low.

According to METZ (ibid.) dams with a mortality in their litter before reaching the age of 90days produced about 39g less milk per day than those whose litter was intact but this difference was not significant. Crossbred dams were heavier at parturition than local dams and produced 71% more milk than the locals. There was therefore a higher rate of reduction in litter size from 0 - 90 days for the local than for the crossbred dams and for the first parity than for the second.

METZ (ibid.) also found that the milk yield persistency of the goats was significantly (p < 0.001) influenced by the environmental effect of parturition month. Measured as milk yield per unit metabolic post-partum weight of the dam, local goats showed higher decrease in milk yield per day of 16.5% during the first phase (0 - 34 days) of lactation than in the second phase (35 - 62 days) and the third phase (63 - 90 days) of lactation. Contrary to this, the crossbred (Katjang x German Fawn) goats showed a gradual increase in decrease of 10.8% and 15.1% during the first and the last two phases of lactation, respectively.

2.1.2 Effect of restricted feeding and realimentation on growth

2.1.2.1 Effect of restricted feeding

Feed restriction reflects seasonal availability of feed which is characteristic of the tropical environment. It is therefore necessary to know the impact of administration of any restriction on animals in general and on growing animals in particular.

Restricted feeding has been found to influence voluntary feed intake, digestibility of feed, growth rate, level of blood constituents and carcass value. Voluntary feed intake later during the first few weeks of realimentation has been found to be low after feed restriction (KEENAN et al. 1970). Restricted feeding caused increased digestibility (HADINOTO, 1984; MARAIS et al., 1991) as a result of reduced rate of passage (MARAIS et al., ibid.; Van BRUCHEM et al.,1994). Lower nitrogen retention has also been associated with restricted feeding (THOMSON et al., 1982).

Reduction in growth rate is inevitable due to reduced energy intake (SEARLE et al., 1972; MARAIS et al., ibid.; SEARLE et al., 1982) and due to increased fat deposition (LEDIN, 1983; WRIGHT and RUSSEL, 1991; SEARLE et al., 1982) especially in growing animals. In some cases, however, subcutaneous fat has been reported to have been lost (LITTLE and SANDLAND, 1975) or deposition of both protein and fat have been found to have reduced (HAYDEN et al., 1993). All this has been reported to influence carcass quality. Where loss in weight was drastic, male animals have been found to be less tolerant to feed restriction than females (CAMPBELL, 1988; ABEBE, 1996; ALLDEN, 1968, 1979). Feed restriction resulted in considerable loss of water (DROUILLARD et al., 1991), decreased blood constituents (SALEM et al., 1989) such as glucose, growth hormones, insulin and thyroxin (NAQVI and KRAF, 1991; HAYDEN et al., ibid.; CHOI et al., 1997; PARK et al., 1994; BURRIN et al., 1990). Some organs like the liver, stomachs and intestines (LEDIN, ibid.; BURRIN et al., ibid.; CARSTENS et al., 1991; FOOT and TULLOH, 1977) tended to reduce in weight while others like the heart (WOLDEGHEBRIEL et al., 1994) tended to increase under the influence of restricted feeding. Although physiological activity in the form of lowered heart and breathing rate has been reported by some workers (WALI and ASHIR, 1990) others reported improved feed efficiency with minimal restriction (WOLDEGHEBRIEL et al., ibid.) while in another case (MARAIS et al., ibid.) the same is reported to have been reduced. Gut fill has been found to increase as a result of restricted feeding (CARSTENS et al., ibid.; TOUKOUROU 1997 ) due to increased consumption of straw (Van BRUCHEM et al., ibid.).


[page 19↓]

In general, animals with higher genetic potential for lean tissue growth have been found to be more sensitive to nutritional stress than those of lower lean tissue potential (CAMPBELL, 1988).

2.1.2.2 Effect of realimentation on growth

Animals subjected to seasonal feed restriction may recover from loss of weight during the rainy season when pasture regenerates or when realimentation is administered. OSBORNE and MENDEL (1916) could show that ‘under suitable dietary conditions lost weight may be regained far more rapidly than during normal growth through the same range of body weight.’

Compensatory growth is defined as increase in growth rate following nutritional restriction (RYAN, 1990; McMANUS et al., 1972) and weight loss (THORNTON et al., 1979). The mechanisms involved in compensatory growth include reduction in maintenance requirement, increased efficiency of growth and fattening, reduction in energy of tissue deposited and increase in feed intake. The effects of these mechanisms on compensatory growth depends on the severity and duration of restriction and the quality of feed during realimentation (ibid.).

Concluding that grazing animals are likely to be subjected to some degree of nutritional restriction that suggests that only about 50% of the growth potential of such animals is realised during the growing phase, RYAN (ibid.) gives an alternative difinition of compensatory growth as follows:

Greater than normal growth rate sometimes observed following nutritional restriction that slows, only maintains or reduces the weight of the animals on which it is imposed for a sufficient enough period of time to allow for adaptation to the lower nutritive state.

The above definition tends to reflect the natural grazing environment characterised by seasonal feed restriction and the season(s) when feed is adequately available. Most cattle and sheep production systems dependent on pasture are subject to wide variations in the weight of the animals due to seasonal fluctuations in herbage production (GINGINS et al., 1980). According to LEDIN (1983) compensatory growth can be used to concentrate growth to periods when feed is available at low cost especially in the Tropics following feed restrictions imposed by the dry season. SALEM et al.(1989), however, point out that for animals reared under extensive conditions in developing countries feed scarcity often experienced during the dry season and during times of drought cannot be counteracted by supplementary feeding because of the scarcity and high prices of concentrated feeds, thus making it largely uneconomical. Nevertheless, prevention of drastic losses in body weight would be desirable. It would be uneconomical to administer supplementary feeding in order to gain weight (FOOT and TULLOH, 1977). The need to minimise costly supplementation implies a necessity to estimate feed requirements for alternative growth patterns (GRAHAM and SEARLE, 1975).

Recurrent quantitative and qualitative seasonal deficiencies in feed supply causing undernutrition and numerical losses of animals are more serious when associated with prolonged droughts (ALLDEN, 1979).

To be put into practice, the ideas mentioned above would demand as pre-requisite, the following:

  1. Knowledge of the level of seasonal feed restriction imposed on the animals by a given environment.
  2. Exact knowledge of the mechanisms of compensatory growth well enough to be able to intervene in time to counteract the adverse effects of seasonal feed restriction.
  3. Felt need to make feed/energy savings in areas not generally affected by seasonal feed restriction.

Compensatory growth may be complete, partial or absent especially in animals in which restriction is imposed soon after birth or at maturity (RYAN, 1990). The duration of compensatory growth would appear to be directly proportional to the level of restriction. Increasing the severity of restriction is more likely to result in compensatory growth being maintained for longer rather than its magnitude being further increased (ALLDEN, 1968; RYAN, ibid.). The exact nature of the relationship between the severity of restriction and the degree of compensatory growth is, however, not clearly understood (RYAN, ibid.). According to McMANUS et al. (1972), the physiological basis of compensatory growth still remains obscure.

The question must also be answered: What is the biological potential of compensatory growth and to what extent could a given level of restriction be justified? It is clear from the above mentioned that [page 20↓]severe restriction may not match the natural ability of the animal to exhibit compensatory growth within the same length of time that restriction took place. It therefore becomes necessary to ‘bring the negative balance forward’ such that, theoretically at least, the mechanism of compensatory growth is seen as having a strong time element. Since the possibility of intensifying the restriction does not always result in any proportionate increase in compensatory growth, to ‘bring the negative balance forward’ could be costly in terms of time and the feed input.

Existing knowledge of the phenomenon of compensatory growth is too insufficient to allow for its control and exploitation for the efficient production of desirable leaner carcasses from meat producing animals (THORNTON et al., 1979). THOMSON et al.(1982) mention that increased appetite and the associated gut fill of animals previously subjected to feed restriction constitute an important factor responsible for compensatory growth.

According to HOGG (1991) compensatory growth lacks a definition that is ‘precise, unambiguous and generally accepted’ and views it as the ability to ‘catch up’ to better fed counterparts. It can at best be described as significantly higher rate of growth per day that an animal fed ad libitum exhibits above that of a genetically identical animal after a period of nutritional stress. Compensatory growth does not invariably occur following some period of nutritional stress or reduced live weight gain. HOGG (ibid.) therefore points out that whereas many trials discuss compensatory growth, the trend has been in fact to report on faster growth rate after a period of restriction because of improved feeding i. e. without any evidence of true i. e. complete compensatory growth.

There is no consensus regarding the causal involvement of the generally accepted mechanisms of compensatory growth (THOMSON et al., 1982) and the factors responsible for inducing it remain undefined (HAYDEN et al., 1993). Neither are the mechanisms by which the so-called stair-step growth influences mammary development and subsequent lactation known (CHOI et al., 1997).

The direction of causality may be difficult to determine in many cases where emphasis is laid on the ‘quantitative relationships between two or more variables’ unless time is considered as an independent variable (FORBES, 1988). The review of literature on growth following realimentation has therefore also tried to reflect on the mechanisms that try to explain this kind of growth, while watching out for any reference to the element of time.

Strong reference to time is also given by HOGG (1991) in the statement: ‘The popular belief that compensatory growth is a unique phenomenon somehow different from normal growth, does not bear close scrutiny. Rather it should be viewed as a transitory period of time, following nutritional stress, during which an animal’s homeostatic mechanisms respond to an increased availability of food. During this transition, large changes occur in hormonal and enzyme levels and activity, maintenance requirement intake and digestibility of food and composition, as well as in the use of partionining of energy and protein.’

It would appear also from the available literature that the length of time within which maximal growth would be expected to occur following realimentation is far much less than expected. Sometimes as short as three weeks (HAYDEN et al. 1993 using crossbred steers); or four weeks (BUTTLER-HOGG, 1984; HOGG, 1991) or five weeks (TOUKOUROU, 1997) in small ruminants at least. FOOT and TULLOH (1977) recorded 30 - 68 days (i. e. about 4 - 9 weeks) in Angus steers such that the individual animals compensating at 30 days had the lowest voluntary feed intake.

Reduction in maintenance requirement: Reduction in maintenance requirement has been associated with changes in the weight of animals as well as changes in requirement that are naturally bound to occur over a given period of time. Reduction in the size of the digestive tract and liver (RYAN, 1990; BURRIN et al., 1990; TOUKOUROU, 1997) and of live weight (SAUBIDET and VERDE, 1976) are directly associated with previous feed restriction (GINGINS et al., 1980; RYAN, ibid.) such that reduction in maintenance requirement per se may not be sustained long enough during realimentation (GRAHAM and SEARLE, 1972; GRAHAM and SEARLE, 1975; DROUILLARD et al., 1991). Reduction in maintenance requirement and faster growth rate during realimentation may therefore not be directly connected. In addition to this, it has been reported that maintenance requirement is not maintained constant (GRAHAM and SEARLE, 1972) over time even when live weight is maintained as much as possible constant (GRAHAM and SEARLE, 1975; FOOT and TULLOH, 1977) and voluntary feed intake and metabolic rate may change according to season of the year (HORTON, 1981; BLAXTER et al., 1982 ; BLAXTER and BOYNE, 1982). Lower energy and N losses (GINGINS et al., ibid.; THOMSON et al., 1982) were attributable to the restriction phase.


[page 21↓]

Increased efficiency of growth and fattening: Growth rate has sometimes been known to occur at faster rate during realimentation even without any corresponding increase in feed consumed (DREW and REID, 1975c). Increased rate of growth and fattening during realimentation (PARK et al., 1987; RYAN, ibid.; WRIGHT and RUSSEL, 1991; CHOI et al., 1997) could be attributable to increased efficiency of protein and plasma-N utilisation (THOMSON et al., ibid.; PARK et al., ibid.). Consequently, increase in amount of water (KEENAN et al., 1970; ) and protein (CHOI et al., ibid.) has been reported by some workers. |Increased efficiency of growth and fattening has been seen as an effort to catch up (THORNTON et al., 1979; HOGG, 1991) especially in cases where animals had lost weight during feed restriction (SAUBIDET and VERDE, 1976) or weight of liver and other internal organs had reduced in size (RYAN, 1990; BURRIN et al., 1990; CARSTENS et al., 1991; TOUKOUROU, 1997). It has therefore not been possible to divorce it completely from previous feed restriction (RYAN, ibid.) from which it is carried over and from the age of realimented animals (RYAN, ibid.; HOPKINS and TULLOH, 1985) or their body weight (CARSTENS et al., ibid.). Maximum reaction has been associated with a feed restriction level of 50% of ad libitum and differences in reaction between male and female lambs have been observed in favour of the former (MARAIS et al., 1991). Nevertheless, weight loss is not always a pre-requisite to efficient growth rate (THOMSON et al., ibid.). Although some workers (VIMINI et al., 1984) have reported that hyperplasial growth is completed during gestation, it is possible based on the results of other workers (CHOI et al., ibid.) to conclude that this could be stimulated with the administration of Low-High feeding.

Reduction in energy of tissue deposited: Reduction in energy of tissue deposited associated with Low-High feeding (BUTTLER-HOGG, 1986; DREW and REID, 1975b) is thought to be the result of an inverse relationship between protein and fat deposition both during the early and later part of realimentation. Low-High feeding has resulted in increased deposition of protein and water (KEENAN et al., 1970; McMANUS et al., 1972; LITTLE and SANDLAND, 1975; LEDIN, 1983; CARSTENS et al., 1991; WRIGHT and RUSSEL, 1991; HAYDEN et al., 1993) especially to replace depleted reserves in the liver and intestines (RYAN, 1990). Indeed faster gain in protein deposition during realimentation may constitute no more than mere response to redress changes taking place in the body during feed restriction (HOGG, 1991). Feed restriction levels below 50% of ad libitum may result in protein deposition being slowed down (see MARAIS et al., 1991) probably due to loss of energy in urine (DREW and REID, 1975c), hence suggesting that extremes of feed restriction could be counterproductive. Replacement of depleted reserves implies the preference of protein and water deposition during the early part of realimentation (DREW and REID, 1975a; LEDIN, 1983; THOMSON et al., 1982) and as long as rapid regain of liver and gut had to be achieved (LEDIN, ibid.). Some workers (THORNTON et al., 1979; DREW and REID, 1975b) could therefore not find any significant difference in final carcass composition ( protein, water, fat and ash) between lambs of High-Low and Low-High treatments. Fat deposition is associated with slow down in growth rate (BUTTLER-HOGG, ibid.). Intramuscular, subcutaneous and kidney fat was found to increase very significantly with High-Low than with Low-High feeding (BUTTLER-HOGG, ibid.). Loss followed by regain of body weight as implied in Low-High feeding caused considerable reduction in carcass fat composition compared with feeding to maintain constant weight (DREW and REID, 1975b). Percentage loss of fat was higher in the offal than in meat (THORNTON et al., 1979). Thus reduction in energy of tissue deposited could be accounted for by increased protein deposition or break down of fat per se or a combination of both.

Increase in feed intake: Realimentation has generally been associated with increased feed intake which at the end of experimentation was lower, higher or equal to ad libitum feeding. Increased intake (GRAHAM and SEARLE, 1975; THORNTON et al., 1979; PARK et al., 1987; NAQVI and KRAF, 1991; PARK et al., 1994) is the result of a big appetite (DREW and REID, 1975c) and has therefore been seen to be influenced by the level of restriction to which realimented animals were previously subjected to (SAUBIDET and VERDE, 1976). Animals allowed to first lose weight and then regain it consumed less feed than those reared at constant weight (FOOT and TULLOH, 1977). Some workers (GRAHAM and SEARLE, ibid.; DREW and REID, 1975c; PARK et al., 1994) have reported increase in feed intake of realimented animals above that of those subjected to ad libitum feeding. It is thought that in such cases, animals were first subjected to drastic weight loss. Total feed consumed by realimented animals may not be significantly different from that consumed by the ad libitum fed group (HADINOTO, 1984) since increased intake per se may be limited to the realimentation phase alone (PARK et al., 1987). Since peak feed intake took place three weeks after administration of realimentation (KEENAN et al., 1970; FOOT and TULLOH, ibid.; HAYDEN et al., 1993) it is reasonable to expect that, initally, feed intake could be associated with increased digestibility (GRAHAM and SEARLE, 1975; HADINOTO, ibid.). Inially high feed intake at the beginning of realimentation reduced as body weight started to increase (MARAIS et al., 1991).


[page 22↓]

It is for the most part difficult to draw a distinct line between the different factors said to be responsible for increased growth rate during realimentation: the role of one may just be a reflection of the beginning, progression or end of one and the same thing. For example reduced maintenance requirement reflects reduced body weight or organ size, reduced body weight or organ size must be quickly regained, regaining of normal organ size is basically a low energy input process, and doing this at a time when animals have previously been subjected to hunger implies high feed intake. Any reasonable and worthwhile approach to this subject should therefore consider its application to solve practical problems of management.

2.1.3 Physiological reaction to high ambient temperature

High ambient temperature has a negative effect on productivity. This negative effect is direct in the form of stress suffered by the animal and the diversion of energy from the purpose of production to regulation of body temperature and indirectly by affecting the availability of feed resources upon which production is dependent. The availability of feed resources has a seasonal pattern implying that they are quantitatively and qualitatively inadequate during some seasons of the year. All this raises the question of the feasibility of rearing crossbred lambs with regard to both survival and maintenance of high productivity. Whereas measurement of productive adaptability needs a long period of time to quantify, basic indication of the ability to survive can be deduced from the physiological reaction of animals subjected to high ambient temperature in a climate chamber.

2.1.3.1 Effect on body temperature

High ambient temperature in the tropics is a major constraint to rearing of high production animals from the temperate region (MISCKE, 1977; KLEIN, 1984; ZIEGLER, 1988; SCHAFFT, 1993). An animal has to maintain a stable state (homeostasis) of its internal environment (e.g. oxygen partial pressure, body temperature, osmotic pressure, pH value and concentration of electrolytes) in order to maintain normal function inspite of the constantly changing state of the physical environment outside (BIANCA, 1971, 1964). In thermoregulation, the adverse effects of climate and weather are compensated for as the animal reacts to maintain normal body functions, which involves considerable expenditure of energy that would otherwise be used for productive purposes (BIANCA, 1971, 1976, 1977). This involves panting, sweating, vasodilatation and -constriction, control of level of intensity of metabolic activity and red blood cell concentration (BIANCA, 1971, 1964). Frequent defecation has also been identified as another form of reaction in response to high ambient temperature (BEAKLEY and FINDLAY, 1955). Lower critical temperature (-3°C for sheep in general; 29°C at birth and 28°C if shorn) is the level of ambient temperature at which the organism will produce heat in order to prevent a fall in body temperature (BIANCA, 1971). Mean critical temperature has been found to be lower at maintenance than at fasting level of feeding BLAXTER and WAINMAN (1961). The higher the milk yield or fattening performance and consequently the higher the feed intake; and the thicker the coat cover and layer of subcutaneous fat, therefore the lower the level of lower critical temperature (BIANCA, 1968). GRAHAM et al.(1959), however, concluded that even closely shorn sheep had difficulties losing heat at ambient temperature levels above critical temperature.

With increasing ambient temperature, loss of heat from the body increasingly takes place through evaporation of water in the form of sweat and panting through the damp respiratory tract (BIANCA, 1968). Sweating, though increasing water intake, does not involve as much energy as panting and does not therefore interfere with feed intake, neither does it present any danger of respiratory alkalosis as panting does. Sweating can result in loss of nutrients (JOSHI et al., 1968). It has also been reported (BIANCA, 1968) that unlike cattle, sheep and goats can only produce little amounts of sweat.

The effects of high ambient temperature have been measured on various species of animals which included many experiments involving sheep. High ambient temperature has been found to result in rise in rectal temperature (McLEAN, 1963; KLEIN, 1984; STELK, 1987; WALI and ASHIR, 1990; STEIN, 1991; KAISER, 1992; MULLER et al. 1994b), and low ambient temperature in a reduction in rectal temperature (STELK, ibid.). Initial reaction to high ambient temperature may be associated with higher rectal temperature than at later stages (BEAKLEY and FINDLAY, ibid.). Increase in relative humidity combined with high ambient temperature has also been associated with increase in rectal temperature (WHITTOW and FINDLAY, 1968; HIPPEN, 1979; STEIN, ibid.; ). Rumen temperature has also been reported to rise with increase in rectal temperature especially in combination with increased metabolic rate as a result of a high level of feeding (STEIN, ibid.). Increased heat production at low ambient temperature of 15°C has been seen as response to increased energy requirement for the regulation of body temperature (KAISER, 1992). Heat stressed animals have shown increased [page 23↓]concentrations of cortisol (MULLER et al. 1994b); and decline in thyroid activity and thyroid hormone secretion (JOHNSON, H and RAGSDALE, 1960; FAICHNEY and BARRY, 1986) that may not be associated with any adjustment to high ambient temperature (YOUSEF et al., 1967). High concentrations of prolactin have been associated with relief from heat stress (PETERS and TUCKER, 1978; FAICHNEY and BARRY, ibid.).

At 15°C ambient temperature, heat loss through vaporisation of water has been found to be normal in steers compared with 35°C (BLAXTER and WAINMAN, 1961). High ambient temperature has been associated with a reduction in the difference between inner body temperature and that of the extremities (WHITTOW, 1962; McLEAN, ibid.). Diurnally alternating ambient temperature may have the same effect as the mean of both extremes maintained constant (GROSSMANN , 1983). However, alternating warm-cold ambient temperature during the day and at night, respectively, had a compensatory rise in rectal temperature during the cold night accompanied by cardiac acceleration and declining skin temperatures, thus implying peripheral vasoconstriction (BIANCA and NÄF, 1977). The adverse effects of high ambient temperature may be clearly manifested between 1100 - 1500 hrs of the day (MULLER et al. 1994c; refer also to PANT et al., 1985).

Table 2: Effects of increasing ambient temperature on the rectal and rumen temperature of wethers fed a low, average and high energy ration.

Rectal temperature

Temperature at 60%RH

Ration 1:

Concentrate:

Roughage:95:5

Ration 2:

20:80

Ration 3:

10:90

From

To

Increase/Decrease (°C)

15°C

30°C

0.8

-

0.5

30°C

35°

0.6

0.6

0.7

15°C

35°C

1.4

-

1.2

20°C

30°C

 

0.3

 

20°C

35°

 

0.9

 

Rumen temperature

15°C

30°C

0.3

.

1.2

30°C

35°C

0.4

.

0.1

15°C

35°C

0.7

.

1.3

20°C

30°C

-

.

-

20°C

35°C

-

.

-

Source: Based on KAISER, 1992, Fig. 12, p. 55

Therewas a rise in rectal temperature with increase in ambient temperature independent of the level of feeding. Rectal temperature tended to be higher with a high raw fibre ration than with a high concentrated feed ration. The opposite was true with regard to rumen temperature and therefore reflect the high level of fermentation in animals that consumed more energy.


[page 24↓]

2.1.3.2  Effect on breathing rate

High ambient temperature has been known to affect breathing rate by accelerating it (McLEAN, 1963; HALES and WEBSTER, 1967; WHITTOW and FINDLAY, 1968; BIANCA, 1971; MIESCKE, 1977; KLEIN, 1984; FAICHNEY and BARRY, 1986; MATHERS et al., 1989; STEIN, 1991; KAISER, 1992) by up to ten times or more; and by reducing it (BIANCA, 1971) if the amount of stress increased further. The first phase of reaction involving accelerated breathing is accompanied by reduction in tidal volume. The second phase in which breathing rate reduces is accompanied by increase in tidal minute volume and can cause death as a result of respiratory alkalosis when C02 has been removed from the lungs into the blood stream, thus causing blood pH to rise (BIANCA, ibid.). Accelerated breathing was mostly effective in reducing body temperature in the upper respiratory tract (HALES and WEBSTER, ibid.).

Increase in breathing rate may not be accompanied by any increase in rectal temperature at high ambient temperature (FAICHNEY and BARRY, 1986). Panting may not therefore be an exact response to high ambient temperature so as to balance heat production with heat loss and may serve only as an approximate means of adjustment (BLIGH , 1959). Panting may also not be an efficient means of increasing heat loss in hot dry climates where the air temperature is usually high since an increase in breathing rate is usually associated with some increase in respiratory ventilation, although this tends to be limited by a reduction in tidal volume (McLEAN, 1963).

Breathing rate may also be affected by feeding level, type of coat cover and psychological status of the animals. Breathing rate was found to be higher for the higher quality pellet diet than for the alkali-treated barley straw supplemented with urea, sulphur, minerals and vitamins (MATHERS et al., ibid. ). A higher roughage diet in the form of alfalfa has been confirmed to cause higher oxygen consumption by the portal drained viscera (gastrointestinal tract, pancreas, spleen and mesenteric fat) and liver of heifers than a concentrated feed of a similar concentration (REYNOLDS et al., 1991).

From 15°C/60%RH to 35°C/60%RH ambient temperature, KAISER (1992) recorded highest increases in breathing rate if change in ambient temperature was administered in combination with a high fibre or a high concentrate ration.

For a given total loss of water from the body in the form of vapour, the presence or absence of a fleece did not influence breathing rate (BLAXTER et al., 1959). Only sensible heat losses at levels of temperature above critical showed wide variations in breathing rate associated with fleece length. Sheep with fleeces were found to have very wide thermoneutral zones, unlike closely shorn ones (ibid.). And for excited sheep even in the resting state, considerable variability in breathing rate has been reported (HALES and WEBSTER, 1967).


[page 25↓]

Table 3: Effect of increasing ambient temperature on breathing rate of wethers fed a low, average and high energy ration

Breathing rate (No. Per minute)

Temperature at 60% RH

Ration 1:

Concentrate:

Roughage:95:5

Ration 2:

20:80

Ration 3:

10:90

15°C

15

-

23

30°C

62

50

123

35°C

156

77

164

20°C

-

15

-

From

To

No. of times of increase above original level

15°C

30°C

4.13

-

5.35

30°C

35°C

10.40

5.13

7.13

20°C

30°C

-

3.33

-

Source: Based on KAISER, 1992, Fig. 12, p. 55

At ambient temperature of between 15°C and 30°C increasing the energy content of the diet was accompanied by an increase in breathing rate. At 35°C both extremes of diet i. e. high fibre content diet and high concentrated feed diet recorded much higher levels of breathing rate than for the average diet treatment, in that order. Raising the ambient temperature from 30°C to 35°C caused a more than 10-fold and 7-fold increase in breathing rate of wethers fed the high fibre and high concentrated feed diets, respectively.

2.1.3.3 Effect on feed intake and digestibility

Feed and water intake: Feed intake forms the basis of production such that a thermostatic regulation of intake (increase and reduction of appetite in the cold and heat, respectively) under extreme climatic conditions becomes an important point for animal production (BIANCA, 1971). High productivity is associated with a high metabolic rate and hence high heat production (BIANCA, 1976). Heat induced reduction in appetite is therefore useful as a mechanism of thermoregulation with the disadvantage that this implies loss in production (ibid.). High ambient temperature is known to cause reduction in feed intake (DAVIS and MERILAN, 1960; STELK, 1987; KAISER, 1992; ) but increased digestibility due to reduced rate of passage (DAVIS and MERILAN, ibid.; BLAXTER and WAINMAN, 1961; McDOWELL et al., 1969; FAICHNEY and BARRY, 1986; STEIN, 1991) and therefore reduction in energy lost in faeces (GRAHAM, 1959); as well as increase in water consumption (McDOWELL et al., ibid.; KLEIN, 1984; FAICHNEY and BARRY, ibid.; STELK , 1987) and frequency of consumption (MIESCKE, 1977). On the other hand, low ambient temperature has been known to increase feed intake (KLEIN, ibid.) followed by reduction in digestibility (CHRISTOPHERSON, 1976; KENNEDY and MILLIGAN, 1978; KENNEDY et al., 1978) and N-balance (BAILEY, 1964). A combination of low ambient temperature and high fibre ration has been associated with a negative balance of both N and energy (KAISER, ibid.). Further, sheep are said to be able to better tolerate high ambient temperature than cattle with regard to loss of appetite due to low metabolism per unit surface area (BLAXTER and WAINMAN, ibid.). The effect of ambient temperature on digestibility has not been found to be lineal (STEIN, 1991).


[page 26↓]

Table 4: Effect of increasing ambient temperature (°C) on DM and raw fibre intake

Temperature at 60% RH

DM intake (%)

From

To

Ration 1:

Concentrate:

Roughage:95:5

Ration 2:

20:80

Ration 3:

10:90

15°C

30°C

-10.81

-

-4.98

30°C

35°C

-11.30

-12.85

-4.44

20°C

30°C

-

-2.95

-

20°C

35°C

-

-15.42

-

15°C

35°C

-20.89

-

-9.20

From

To

Raw fibre intake (%)

15°C

30°C

-15.26

-

-10.00

30°C

35°C

-14.91

-13.33

-1.59

20°C

30°C

-

-8.16

-

20°C

35°C

-

-20.41

-

15°C

35°C

-27.90

-

-11.43

Source: Based on KAISER, 1992, Fig. 3, p. 30

The above table shows that both DM and raw fibre intake were negatively affected by high temperature. The reduction in intake of raw fibre was generally higher than for DM. The reduction in intake of both nutrients was lower for Ration 3 than for 1 and 2.

2.1.3.4 Effect on production

High ambient temperature has been associated with a reduction in growth rate and milk yield. Data on growth performance of lambs under high ambient temperature in experiments in the climatic chamber is limited in terms of how it affects various genotypes. Data on milk performance has largely been concerned with dairy cattle. The physiological reaction of cattle in this case could be comparable to that of sheep subjected to similar conditions.

From 15°C/60%RH to 30°C/60%RH constant ambient temperature, 55% reduction in ADG has been reported and associated with 38% reduction in energy intake (STELK, 1987). In this case, two-thirds of the lambs reared at 30°C /60%RH were prematurely removed from the experiment for failing to meet the minimum growth requirement of 50g Average Daily Gain. The same lambs recorded an average of 0.4°C higher rectal temperature than those retained.

High ambient temperature has been associated with decline in milk yield (JOHNSON et al., 1960; SCOTT and MOODY, 1960; WAYMAN et al., 1962; MIESCKE, 1977; RODRIQUEZ et al., 1985; KLEIN, 1984; BURMEISTER, 1988; ZIEGLER, 1988) especially in late lactation (JOHNSON et al., ibid.). Decline in milk yield was the result of reduced feed intake (WAYMAN et al., ibid.; MIESCKE, 1977) and reduced efficiency of utilisation (WAYMAN et al., ibid.) and could be accompanied by loss in weight (BURMEISTER, ibid.) and reduction in fat content (ZIEGLER, ibid.). At high ambient temperature, interaction with feeding level was increased with regard to milk yield and quality and the physiological reaction of dairy cattle (SCOTT and MOODY, ibid.; LEIGHTON and RUPEL, 1956; WAYMAN et al., 1962). At ambient temperature of between 15°C and about 22°C, milk yield was less sensitive to variation (CUMMINS et al., 1992).


[page 27↓]

High milk yield under high ambient temperature conditions was associated with low body temperature and high sweating rate (KLEIN, 1984). At about 40°C, high productivity was associated more with high energy deposition in the form of fat due to heavier body weight and higher chronological age (ZIEGLER, 1988). Earlier, JOHNSON et al. (1960) noted that potentially high and average milk yielding dairy cattle may demonstrate similar performance at an extreme ambient temperature level of 90°F/50%RH.


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