[page 82↓]

6  Discussion

6.1 Comparative pre-weaning growth performance of lambs of different genotypes

The weight measurements show that the pre-weaning growth performance of the crossbred lambs was superior to that of the pure Cameroon. Although the difference between the C1 and the C2 crosses was not significant, the pre-weaning growth performance of the C2 crosses was always superior.

Crossbred lambs (C1 and C2) exhibited significantly heavier birth weight and pre-weaning ADG than the Cameroon lambs. The C1 had a significantly heavier birth weight than the C2. From birth to weaning, however, the C2 recorded higher weight gain than the C1but failed to be significant at p 0.05.

The superior performance of crossbred lambs (C1 and C2) over pure Cameroon lambs with regard to live weight and growth rate is supported by results from literature indicating that crossbreeding of local tropical breeds with exotic ones selected for high productivity does increase animal productivity under tropical conditions. Increased performance has been in the form of higher ADG, birth weight, weight at slaughter and feed efficiency (NGERE, 1973; NGERE and ABOAGYE, 1981; MOHAN et al., 1985; FERNANDES and DESHMUKH, 1986; ABEBE, 1996; GATENBY et al., 1997). However, in order to avoid problems of adaptation to the tropical environment of animals with a temperate origin (BIANCA, 1976) or their crosses for that matter, preliminary tests to determine their suitability may have to be done where possible. The trend of the C2 to achieve relatively heavier weights and higher ADG during the pre-weaning phase may also suggest their suitability under tropical conditions where the availability of feed is seasonal and thus not ensured throughout the year. In such a situation, a combination of fast growth at a time when milk is readily available might help to offset the adverse effects of seasonal feed restriction later. Average Daily Gain estimated from 0 - 30, 30 - 60 and 60 - 90 days of age during pre-weaning growth showed the growth pattern of the crosses to be characterised by highest values from 0 - 30 days of age and lower values thereafter whereas that of the Cameroon tended to be constant during all three phases. As already mentioned in the case of the C2 crosses, high ADG values during pre-weaning growth combined with high milk performance of the ewe would constitute suitability for production in environments characterised by natural feed restriction.

6.2 Milk yield performance of Cameroon and C1 ewes using the suckling method

The milk yield experiment using the milk suckling method shows that the C1 ewes could yield 2 - 3 times more milk than the pure Cameroon ewes. The milk yield of the C1 ewes was highest at the beginning of lactation but declined gradually up to weaning. The Cameroon had a lower level of performance with higher persistence. The high milk performance of the C1 ewes at the beginning of lactation might increase the pre-weaning growth rate of the lambs. The model fails to explain most of the variation after the third week and there is a double increase in the coefficient of variation during this period. It appears that the number of animals considered was too small to ensure adequate analysis of data after the third week of lactation.

METZ et al.(1985) and METZ (1990) also recorded higher (p = 0.001) milk yields in C1 crosses between the local Malaysian Katjang goat and the German Fawn over the pure Katjang. The lactation behaviour of the C1 ewes up to nine (9) weeks in the current work does not confirm the statement by PETERS and LAES-FETTBACK (1995) that higher milk yield is associated with lower persistence. It would appear that the length of lactation in the current work was not long enough to prove this point.

Though recommended as a suitable method to study the lactation of non-dairy breeds of sheep, the lamb-suckling method is tedious and time-consuming and involves excessive handling that could affect milk let down (COOMBE et al., 1960). For the sake of accuracy, most studies employing this method have been carried out over the first 10 weeks of lactation. Comparison of the lamb-suckling method and oxytocin (5 IU) followed by hand milking showed very significant (p < 0.01) increases in yield using the latter method over a period of 10 weeks. According to OWEN (1957) separation of the lamb from the ewe may have the effect of depressing milk yield. DONEY et al. (1979) compared milk yield of ewes using the suckling and oxytocin methods and recorded a [page 83↓]highly significant (p < 0.001) difference in favour of the latter. The difference was more pronounced during the first week of experimentation most likely due to failure by the lambs to consume all the milk available. Milk consumption by twin lambs and single lambs tended to be similar during the first week. PEART et al.(1972) found that differences in milk yield of single-, twin-, triplet- and quadruplet-suckled ewes occurred during the first three weeks of lactation. The lamb-suckling method though implying the problem of residual milk (up to 23% compared with the oxytocin method according to POULTON and ASHTON, 1972) is all the same still regarded as practical for non-dairy ewes because it reflects milk consumption (WALLACE, 1948; OWEN, 1957; COOMBE et al., 1960).

The milk yield of the C1 and the Cameroon ewes referred to here does not therefore reflect potential yield. It is, however, important as a reflection of milk consumption of the lambs which was higher for the C2 lambs than for the Cameroon ones. The higher pre-weaning growth rate of the C2 crosses already discussed above should have presented an added advantage compared with the pure Cameroon.

6.3 Comparative post-weaning growth performance of lambs of different genotypes

6.3.1 Influence of different feeding levels on performance

Post-weaning growth performance: Subjecting one group of lambs to High-Low and Low-High feeding caused significant differences in end-weight and ADG over the whole experimental period. Low-High feeding resulted in significantly higher end-weight and ADG over the whole experimental period than High-Low feeding. Treatment did not affect intake of total energy over the whole experimental period, however, lambs subjected to Low-High feeding consumed highly significantly more than those subjected to High-Low feeding during 7 - 12 weeks. Thus Low-High feeding was associated with some compensatory growth which was largely the result of increased intake of energy from concentrated feed during the High phase of feeding, more especially during 8 - 12 weeks. Intake of concentrated feed was not significantly different between treatments during Week 7 despite significantly higher ADG still in favour of the Low-High treatment.

The C1 and C2 crosses had significantly higher ADG than the Cameroon. The C1 crosses had higher ADG than the C2 although the difference between them failed to reach significance at p 0.05. The C2 achieved higher ADG than the C1 during the first six (1 - 6) weeks of the experiment although the difference between them was not significant. The C1, however, had significantly (p 0.01) higher ADG than the C2 during the last six (7 - 12) weeks of the experiment. Thus the C2 could still exhibit higher growth rate than the C1 during the early part of post-weaning growth. This trend was also observed during pre-weaning growth. In terms of end-weight, the C2 crosses were still heavier than the C1 although the difference between them failed to reach significance at p 0.05. Thus the heavier live weight of the C2 crosses during pre-weaning growth put them at better advantage than the C1 crosses during post-weaning growth that was characterised by the Low phase of feeding.

Although intake of total energy by C1 and C2 crosses was significantly higher than that by the Cameroon and that by the C2 significantly higher than that by the C1 due to differences in live body weight on the basis of which feed restriction was calculated, intake of energy above maintenance level, though not significant, was highest for the Cameroon, followed by the C2 crosses and lastly, by the C1 crosses.

Relative intake of wheat straw by the C1 and C2 crosses was higher than by the Cameroon during the High phase of feeding but this was the reverse during the Low phase of feeding. Increased consumption of wheat straw by the C1 and C2 crosses seems to have been tied up with a higher level of feeding.

With regard to the combined effect of both feeding level and genotype, the C1 and C2 of both feeding treatments recorded significantly higher end-weight than the Cameroon. The difference in end-weight between the Cameroon of both feeding treatments was not significant; that between the crosses of both feeding treatments was also not significant. However, lambs in the Low-High treatment group had comparatively higher end-weight than those of the High-Low one. The C1 and C2 of the Low-High feeding treatment achieved the highest ADG values in that order; followed by the C1 of the High-Low treatment and the Cameroon of the Low-High treatment; and lastly the C2 [page 84↓]and Cameroon of the High-Low treatment, again in that order. The close comparison between the latter two genotypes is due to the heavy loss in weight suffered by the C2 following change-over to the Low phase of feeding. The combined effect of both feeding level and genotype on energy intake per kg metabolic weight and energy intake above maintenance was not significant. However, the Cameroon lambs and those of the Low-High feeding treatment tended to have a higher level of consumption than the others.

Carcass evaluation: Treatment caused no significant difference in the weight of muscle, fat and bones of the seven selected valuable cuts although the lambs subjected to Low-High feeding demonstrated compensatory growth tended to record higher values than those subjected to High-Low feeding except for weight of fat and bones of the middle where it was the reverse. Cooling losses were significantly higher in the Cameroon than in the C1 and C2 crosses. Significant weight increases in response to compensatory growth were largely limited to the digestive organs (except for weight of omasum) i. e. intestinal tract, intestines, rumen, reticulum, abomasum, liver, pancreas, pharynx; as well as blood and lungs.

Low-High feeding and subsequent compensatory growth was associated with significantly higher fluid content. The fluid content of the C1 was significantly higher than that of the C2 and the Cameroon and that of the males significantly higher than that of the females.

Compared with the Cameroon, the C2 produced the heaviest carcass half followed by the C1. Wide differences between the C1 and the C2 crosses occurred in favour of the C2. The C2 would thus seem to be the animal of choice where post-weaning rearing involved an element of Low feeding.

Females had a heavier carcass weight than males although the difference between them failed to be significant at p 0.05.

Relative weight of carcass part to weight of Right Half and weight of muscle and fat to carcass part from which it was dissected showed some variation largely due to genotype and sex. The effect of treatment, genotype and sex on carcass quality was mainly not significant except with regard to fluid content for which significant (p 0.05) effect of treatment, genotype and sex was recorded in favour of the Low-High, C2 crossbred lambs and males; and with regard to light reflection score for which the C2 recorded a significantly (p 0.05) higher value than the C1 crosses. SAñUDO et al. working with light (10 - 12kg) and medium weight carcasses of Rosa Aragonesa, Lacaune and German Merino lambs could not confirm any increase in carcass weight that could either be associated with any relative increase in the amount of fat or reduced amount of muscle due to an allometric relationship in the growth of these tissues. The same workers, however, associated increased fluid content (excudativeness) with late maturity. Their association of lower growth rate in the Rosa Aragonesa with a darker colour due to a higher quantity of pigments as a result of increased fat deposition that causes a lower transfer of oxygen to the muscles, disagrees with current findings in which the C2 recorded significantly (p 0.05) higher reflection score than the C1 despite the tendency of the latter to exhibit higher post-weaning growth.

Considered over the whole twelve (12) week period, Low-High feeding did not involve any significant increase in feed consumption. This agrees with the findings made by HADINOTO (1984) and may be explained by the fact that growth hormone levels (DRIVER and FORBES, 1981; HEYDEN et al. 1993) as well as those of plasma urea (HEYDEN et al., ibid.) are higher during the Low phase than during High phase of feeding. It somewhat disagrees with results of other workers (KEENAN et al., 1970; McMANUS et al., 1972; DREW and REID, 1975c; THORNTON et al.;1979) most likely because animals in these experiments often suffered considerable weight loss. Nevertheless, fast growth rate demands increased feed intake whose increased efficiency of utilisation might be enhanced as a result of efficient function of the digestive system immediately after the restriction phase (KEENAN et al., ibid.; HADINOTO ibid.).

The literature considered so far does not mention or define any short period phenomenon with influence on feed intake, metabolic activity or growth of animals. HOGG (1991) stresses, however, that compensatory growth should not be seen as a phenomenon completely different from normal growth. And RYAN (1990) concludes that maintenance requirement during realimentation seems to vary from time to time. HAYDEN et al (1993) describe the occurrence of compensatory growth during realimentation as transient. DRIVER and FORBES (1981) recorded periodic peak secretions of growth hormone (GH) whereby GH levels tended to be high at times of feed intake; and feed [page 85↓]removal for a period of ten (10) hours prompted an increase in the size and frequency of GH peaks.

The intensity of growth rate indeed varies from time to time as the current work shows. This is a natural reaction of the animals which in the natural state reflects times of abundant feed availability when higher weight gains could be made and times of seasonal feed restriction when only little gain could be made or when they could even suffer a loss in live body weight. At least from a practical point of view, some workers (PARK et al., 1987; SALEM et al., 1989; PARK et al., 1994; CHOI et al., 1997) have already conducted experiments that seem to exploit this very phenomenon. An effective balance between recurrent restriction and realimentation involving alternating 10-day periods has been reported by SALEM et al., ibid.). There was a trend, however, for compensating animals not to recover their weight over the period of treatment. This may be explained by the short duration of the restriction phases. THOMSON et al.(1982) associated a long period of restriction with increased duration of compensatory growth later.

ALLDEN (1968) showed that lambs reared on a Low-High plane of nutrition during the first and second six months of life, respectively, weighed heavier than those on the High-Low plane i. e. they exhibited compensatory growth. The interest in compensatory growth has also been from the point of view of producing leaner carcasses (THORNTON et al., 1979). The heavy increases in the weight of the digestive organs were followed by moderate but proportionate increases in the growth of muscle, fat and bones. WRIGHT and RUSSEL (1991) also recorded a trend for heavier weight of bone of realimented Charolais crossbred steers compared with the ad libitum group.

A case of proportionality (see TOUKOUROU, 1997) can thus be said to have taken place with regard to the growth of the seven selected economically valuable parts in the current work. The non-compensating group (1) subjected to High-Low feeding was not characterised by any increase in fat deposition such that a reduction in the energy balance of body organs for the compensating group is not evident here. According to KIRTON and JOHNSON (1979) bigger carcasses tend to record bigger measurements in general and with regard to weight of fat as well. Some workers have reported increased protein deposition (KEENAN et al., 1970; McMANUS et al., 1972; DREW and REID, 1975a; THOMSON et al., 1982; LEDIN, 1983; RYAN, 1990) and water (McMANUS et al., ibid.; LEDIN, ibid.) during the High phase of feeding. At the completion of realimentation i. e. High phase of feeding, no significant difference could be found between the body composition of realimented animals and those fed ad libitum (DREW and REID, 1975a; DREW and REID, 1975b). MARAIS et al.(1991) could also not associate realimentation following exposure to variable levels of feed restriction with any significant differences in protein content expressed as a percentage of body weight. The fat protein: protein ratio generally remained constant for all treatment groups (80%, 65% and 50% of ad libitum) except in the case of the 50% group where it was slightly high due to reduced rate of protein deposition. Nevertheless, protein deposition was reported to have increased upon realimentation. A proportionate increase in the body constituents of compensating animals has two phases: firstly one in which there is an increase in the proportions of protein and water deposited, and secondly, one in which there is an increase in fat deposition while that of protein and water reduces (WRIGHT and RUSSEL, 1991). HAYDEN et al.(1993) also associated increase in empty body weight and empty body protein with the initial phase of compensatory growth.

Compensatory growth is the result of a Low-High level of feeding. But any level or length of restriction that results in loss of weight and overtaxes the natural ability of the animal to gain weight would not tenable in management practice.

High and low profiles of growth rate should be further studied and correlated to the mechanisms of compensatory growth as currently defined. Maximal compensatory growth potential/response if properly documented and clearly understood should help to avoid the administration of restricted feeding that overtaxes the natural ability of the animals to regain weight. It should also enable feeding intervention to be made in the form of timely supplementation to avoid loss of animals where seasonal lack of feed becomes drastic or is prolonged.


[page 86↓]

6.3.2  Reaction to high ambient temperature during the day and of alternating feeding levels

Post-weaning growth performance: Subjecting Cameroon and C2 crossbred male and female lambs to High-Low High feeding i. e. three feeding levels lasting four weeks each and at 31°C/50%RH during the day and 15°C/70%RH at night caused no significant differences in ADG over the whole experimental period. However, the effect of genotype on ADG was significant during the second High phase of feeding (9 - 12 weeks). The C2 crosses which had significantly higher ADG than the Cameroon during this period were thus still able to react positively to the Low-High component despite the ambient temperature level they were subjected to. Significantly higher ADG of the C2 crosses compared with the Cameroon during 9 - 12 weeks was accompanied by significant increase in intake of total energy and concentrated feed. However, the intake of energy above maintenance level was significantly higher for the C2 crosses than for the Cameroon during 1 - 4 weeks and over the whole experimental period. Decisive for the significantly higher intake of energy above maintenance by the Cameroon compared with the C2 crosses were the first four weeks during which the latter recorded more refusals of concentrated feed and tended to consume more wheat straw. After this time, intake of concentrated feed by the C2 crosses does not seem to have been more affected than intake by the Cameroon.

Compared to the previous two experiments sex differences seem to have sharpened with regard to ADG as a result of the heat treatment with male lambs gaining significantly more than female ones during 9 - 12 weeks. This was associated with significantly higher amount of feed being consumed by males than by females during this time.

The C2 showed a higher and more stable absolute consumption of wheat straw than the Cameroon but relative wheat straw consumption of the Cameroon was higher than that of the C2 crosses during the Low and second High phases of feeding. The feeding behaviour of the lambs at 31°C/50%RH during the day and 15°C/70%RH at night was observed to be different from those reared at stall ambient temperature conditions. They required much more time to finish their daily ration of concentrated feed during the day at 31°C/50%RH than those reared at stall ambient temperature - apparently a behavioural mechanism to avoid a rapid rise in body temperature. As a result of this, the trend was to consume more straw. Concentrated feed, DM and energy intake of the males was very significantly higher than that of the females. It is clear that the feed intake of the females was more disadvantaged at 31°C/50%RH during the day and 15°C/70%RH at night than that of the males.

Rectal temperature: The C2 crosses had higher values of rectal temperature measured at 1200 hrs with 31°C/50%RH ambient temperature than the Cameroon. Significant differences over the whole experimental period were attributed mainly to the Low feeding phase during 5 - 8 weeks and to the second High phase of feeding during 9 - 12 weeks. Thus extreme (Low and High) levels of feeding sharpened the difference in rectal temperature of the Cameroon and the C2 crosses. Females had higher rectal temperature than males with significant differences occurring during the period of conditioning and during the Low phase of feeding (5 - 8 weeks)

The Cameroon had higher values of rectal temperature measured at 2000 hrs than the C2 crosses. A significant difference occurred during 5 - 8 weeks. Although females had higher rectal temperature than males at this time, a significant difference occurred during 5 - 8 weeks.

Thus differences in rectal temperature measured at 1200 hrs and at 2000 hrs between the Cameroon and the C2 crosses on one hand and the males and the females on the other were widened mainly as a result of the Low phase of feeding (5 - 8 weeks) during which a considerable amount of wheat straw was consumed, and to some extent as a result of the second High phase of feeding (9 - 12 weeks) characterised by increased intake of concentrated feed.

Breathing rate: The C2 crosses had higher breathing rate per minute than the Cameroon. The difference was significant during the period of conditioning, during 5 - 8 weeks and during 9 - 12 weeks. Females had higher breathing rate than the males with significant differences occurring during 5 - 8 weeks and during 9 - 12 weeks. The effect of genotype and sex on breathing rate per minute at 2000 hrs was not significant. Thus measurement of breathing rate was only relevant at 1200 hrs in this case and the difference in breathing rate per minute measured at 1200 hrs between the Cameroon and the C2 crosses on one hand and the males and the females on the other were widened as a result of both the Low (5 - 8 weeks) and the second High (9 - 12 weeks) phases of [page 87↓]feeding during which a considerable amount of wheat straw and concentrated feed was consumed, respectively.

Carcass performance: In absolute terms, the C2 had heavier weight of carcass parts than the Cameroon and males more than the females.

The males produced more than the females in terms of total weight of muscle, fat and bones.

Relative weight of fat to weight of carcass part from which it was dissected generally showed more fat deposition in the Cameroon than in the C2 crosses.

The effect of genotype on carcass quality was limited to pH value of MSM, 24 hours post-mortem for which the Cameroon had a significantly higher value than the C2 crosses.

High ambient temperature has been associated with reduced ADG. Ambient temperatures of around 30°C have been associated with reduced feed intake (STELK, 1987) and more time to reach slaughter weight as a result of reduced ADG (STELK, ibid.) and reduced milk yield (MIESCKE, 1977; BURMEISTER, 1988) and live body weight (BURMEISTER, ibid.) than at lower ones. Changes in some organs have also been reported for example increased weight of heart and reduced weight of liver (STELK, ibid.). Reduced rate of passage has also been reported (FAICHNEY and BARRY, 1986; STEIN, 1991). As a result, digesta content tended to increase (FAICHNEY and BARRY, ibid.). It would therefore be assumed that the lambs subjected to the heat treatment in the current work could not attain their ADG potential. There was, however, a positive response to the Low-High component of feeding during the last 8 weeks of the experiment in which the C2 crosses had significantly higher ADG than the Cameroon. Thus the C2 crosses could, more than the Cameroon, exhibit the phenomenon of compensatory growth even at high (31°C/50%RH) ambient temperature during the day and low (15°C/70%RH) at night - similar to what has been demonstrated by many workers since the time of OSBORNE and MENDEL (1916) to date to be due to Low-High feeding.

It has been considered that alternating ambient temperature made to be high at one time and low at another has the same effect as constant temperature calculated as the mean of both levels (GROSSMANN, 1983). As has been observed in the current work, C2 crosses whose rectal temperature was higher than that of the Cameroon during the day tended to compensate by having lower temperature at night and vice versa. It has, however, also been considered that diurnally alternating high ambient temperature can adversely affect feed intake during the hot temperature of the day (SCHAFFT, 1993). Indeed at 31°C/50%RH during the day and 15°C/70%RH at night, the feed intake of the C2 was adversely affected during the first four weeks compared with the Cameroon lambs. Although intake of concentrated feed by the C2 crosses during 1 - 4 weeks may reflect problems of adjustment to high ambient temperature of 31°C/50%RH, HAYDEN et al. (1993) viewed a similar period of time (i. e. 34 days) to be the necessary period of adjustment for DMI of steers fed on ad libitum and restricted basis. FOOT and TULLOH (1977) considered the first three weeks as a period of acclimatisation after observing that maximal feed intake of realimented Aberdeen-Angus steers was reached within three weeks after which it became steady.

Intake of concentrated feed in the previous two experiments in 1994 and 1995 was observed to last only about 15 - 20 minutes unlike in the case of the heat treatment where this lasted longer and was associated with a rather increased frequency of wheat straw intake. KAISER (1992) reported that the rate of rejection of roughage tended to reduce with high ambient temperatures. MULLER et al.(1994c) found that dairy cows reared at ambient temperature of 25.1°C or above and provided with shed spent significantly (p < 0.05) more time feeding during the day than the no-shed ones except at night when both groups had increased feeding activity than during the day. No-shade cows, however, tended to have more feedings (p = 0.08) during the day than those provided with shade and thus tending to agree with the fact that heat stressed animals will tend to regulate body temperature through reduction of appetite (BIANCA, 1971) and that this will tend to be compensated for by increasing the frequency of feed intake.

The difference in rectal temperature between the Cameroon and the C2 crosses measured at 1200 hrs at high ambient temperature of 31°C/50%RH especially during 5 - 8 weeks when the Low phase of feeding was administered highlights the fact that the inherent difference between the two genotypes and between male and female lambs could be narrowed or widened (as in this case) depending on the level of feeding. Although KAISER (1992) associated an ambient temperature of [page 88↓]30°C with rise in rectal temperature independent of level of feeding, such rise in rectal temperature could be further enhanced in combination with extremes of either low or high levels of feeding characterised by very high or low fibre content. The difference in rectal temperature of the Cameroon and the C2 crosses measured at 1200 hrs with high ambient temperature of 31°C/50%RH during 9 - 12 weeks when the High phase of feeding was administered shows how this difference could also be widened by high level of feeding, though to a lesser degree, compared with the effect of the Low phase of feeding already mentioned. STEIN (1991) and KAISER (1992) confirmed the effect of high (30°C and 35°C) ambient temperature in combination with a high concentrated feed ration to be associated with a lower reduction in metabolic rate compared with a combination of high ambient temperature and high fibre ration. On its own, high ambient temperature around 30°C has been associated with increased concentrations of pancreatic glucagon while depressing those of thyroxin (FAICHNEY and BARRY, 1986).

The difference in rectal temperature of the Cameroon and the C2 crosses measured at 2000 hrs at low ambient temperature of 15°C/70%RH especially during 5 - 8 weeks when the Low phase of feeding was administered highlights the same effect of low feeding on rectal temperature already discussed except that it was this time the Cameroon and not the C2 crosses that recorded a higher value. By design, feed intake was low both during the day and at night during 5 - 8 weeks. According to KAISER (1992) increase in high ambient temperature caused reduction in feed intake especially if done in combination with a high fibre ration than otherwise. Conditions of reduced feed intake would imply increased energy expenditure for thermoregulation especially at night when ambient temperature dropped to 15°C which has been associated with negative N retention (KAISER, 1992). At cold ambient temperature (BIANCA and NÄF, 1977; KENNEDY et al., 1982) there is an increase in heat production. Compared with higher levels of ambient temperature, heat production has been found to be highest at cool ambient temperature of 15°C/60%RH in combination with high fibre ration (KAISER, ibid.). Low ambient temperature has also been associated with increased thyroid secretion (HOERSCH et al., 1961; HORTON, 1981). For this reason, rectal temperature at 2000 hrs was generally found to be higher than that at 1200 hrs in the current work. BIANCA and NÄF (ibid.) have reported a compensatory rise in body temperature during the cold night accompanied by cardiac acceleration and declining skin temperatures. In the current work, the C2 crosses recorded lower rectal temperature at 2000 hrs than at 1200 hrs during 5 - 8 weeks when intake of concentrated feed was restricted to 1.5 times above maintenance level.

It is interesting to note that the wide difference in rectal temperature between the Cameroon and the C2 crosses measured at 1200 hrs with high ambient temperature of 31°C/50%RH and at 2000 hrs with low ambient temperature of 15°C/70%RH during 5 - 8 weeks when the Low phase of feeding was administered has also been reflected between the males and the females with the males always recording a lower value. Rectal temperature of the Cameroon that was higher than that of the C2 crosses at 1200 hrs rose to higher than that of the latter at 2000 hrs. Thus rectal temperature of the two genotypes was compensated for between day and night unlike that between males and females which was not.

Suitable rearing of C2 crossbred lambs in general and of female lambs in particular at high ambient temperature of 31°C/50%RH would ideally demand a higher level of feeding than was administered here for the sake of reducing rectal temperature. PANT et al.(1985) found drastic changes in ambient temperature during the day with strong influence on the physiological reaction of the animals.

Although the C2 crosses recorded higher breathing rate per minute than the Cameroon at 1200 hrs with high (31°C/50%RH) ambient temperature, and the females higher than the males under the same conditions, significant differences during the Low (5 - 8 weeks) and High (9 - 12 weeks) phases of feeding show that as with rectal temperature, these differences in breathing rate per minute were highly influenced by the level of feeding. High ambient temperature was found to increase not only rectal temperature and evaporative water loss but respiration rate as well (BUNTING et al., 1992). KAISER (1992) found that raising ambient temperature from 15°C/60%RH to 35°C/60%RH had the effect of causing higher rises in breathing rate per minute and in body temperature when administered in combination with high fibre ration than with high concentrated feed ration. High respiration rate at 32°C ambient temperature has been associated with decreased levels of thyroxin (T4) and triiodthyronine (T3) according to PEARSON and ARCHIBALD (1990) and MATHERS et al. (1989). High ambient temperature per se has also the effect of raising breathing rate per minute. BIANCA and NÄF (1977) associated the change from morning temperature to [page 89↓]afternoon temperature with a 3-fold increase (p < 0.001) in breathing rate. Variation based on coat colour (black and white) was more a feature of goats than sheep. In general, body temperature and pulse rate are known to record lower levels during the early hours of the morning and to reach a peak in the later part of the afternoon (PATCHELL, 1954; see also MULLER et al., 1994b).

At 20°C/80%RH, 20°C/60%RH and 25°C/80%RH STEIN (ibid.), recorded about 3.5 times increase in breathing rate of adult wethers from a level of 14.1 - 15.2 per minute to about 50 per minute at 30°C/60%RH. FAICHNEY and BARRY (1986) could not associate a 4-fold increase in breathing rate at about 30°C ambient temperature with any significant rise in rectal temperature of sheep. However for lactating dairy cattle a level of ambient temperature of 28°C or above is without any doubt already associated with stress conditions (KLEIN, 1984; RODRIQUEZ et al., 1985).

In excited animals breathing rate may show a distorted picture (HALES and WEBSTER, 1967). A reflection of this was observed in the current work for breathing rate measured at 2000 hrs with low (15°C/70%RH) ambient temperature towards the end of the experiment.

The C2 crosses produced bigger carcasses even at high (31°C/50%RH) ambient temperature during the day and low (15°C/70%RH) at night. Relative weight of carcass part to weight of Right Half and weight of muscle and fat to carcass part from which it was dissected showed some variation largely due to genotype and sex and there is nothing to suggest that animals reacted any more different following the heat treatment than they would otherwise have reacted. The Cameroon tended to deposit more fat than the C2 crosses in relative terms.

Almost all the literature referred to above relate to experiments in which animals were exposed to a given constant ambient temperature over a given period of time i. e. not following an alternating daily rhythm. It would, nevertheless, appear that an alternating ambient temperature of 31°C/50%RH during the day and 15°C/70%RH at night would not constitute any constraint, in absolute terms, to rearing of crossbred lambs.

6.4 Pattern of deposition of kidney and mesenteric fat of slaughtered lambs following exposure to different feeding levels under stall ambient temperature conditions (1995) and to high ambient temperature during the day with alternating feeding levels (1996)

With the element of Low feeding in both the High-Low and Low-High feeding treatments, the effect of treatment on the relative amount of total, kidney and mesenteric fat deposited failed to be significant in 1995 although that of kidney fat tended to be higher and that of mesenteric fat lower in the former treatment in which compensatory growth occurred. Relative deposition of kidney fat was significantly higher in the Cameroon than in both the C1 and C2 crosses. Relative deposition of mesenteric fat was significantly higher in both the Cameroon and the C2 crosses than in the C1 crosses.

With High-Low-High feeding in the heat treatment (31°C/50%RH during the day and 15°C/70%RH at night) in 1996, total fat, kidney and mesenteric fat deposited in relative terms was significantly higher for the Cameroon than for the C2 crosses and this shows a similar trend as for the previous experiment in 1995. However, absence of any significant differences between males and females shows an unusual departure from the trend observed during the previous experiment for the same traits. In fact, following the heat treatment, the amount of mesenteric fat deposited in relative terms was higher in the males than in the females although not significantly different. Thus the heat treatment reversed the trend in the deposition of mesenteric fat between males and females unlike the case in the previous experiment at stall ambient temperature. The Cameroon and the male animals form the category of animals that recorded lower rectal temperature at 1200 hrs and consequently lower breathing rate compared with the C2 crosses and the female lambs. Males in the heat treatment also consumed significantly more concentrated feed, DM and total energy than the females. Internal fat deposition and more especially deposition of mesenteric fat would therefore appear to have implications for thermoregulation. Since the deposition or mobilisation of any sizeable reserves of fat would require a considerably long period of time, such implications for thermoregulation could therefore only be expected to be long-term in nature.

HAYDEN et al.(1993) found that a 25% decrease (p < 0.05) in kidney, pelvic and heart fat depot was associated with restriction followed by realimentation. MARAIS et al.(1991) further found that the fat content of ewe lambs expressed as percentage of live weight was higher than that of ram [page 90↓]lambs. The difference in the pattern of fat deposition between males and females should explain to a large extent the findings of other workers (see ALLDEN, 1968; CAMPBELL, 1988; ABEBE, 1996) regarding the degree to which males would tend to be less tolerant to feed stress than females.

ZIEGLER (1984) associated increased heat tolerance in dairy cattle not only with advanced chronological and reproductive age, but increased fat deposition as well. It is not clear in the current work, exactly why lower rectal temperature and breathing rate during the heat of the day at 31°C/50%RH should be associated with increased deposition of kidney and mesenteric fat observed in the slaughtered Cameroon and male lambs. Hales (Division of Animal Production Research Report, 1978 - 79) mentions that under moderate heat stress i. e. at 40°C ambient temperature, cardiac output in sheep was not increased but that a marked redistribution of blood was effected that resulted in increased flow to tissues of the skin and the respiratory muscle at the expense of flow to the non-respiratory muscle and abdominal organs. It would therefore appear that the increased deposition of fat in the kidney and the intestines is favoured at high ambient temperature so that it does not interfere with thermoregulation in the respiratory organs and the subcutaneous region. This may constitute, at least to some extent, the mechanism of thermoregulation through homeorrheris mentioned by other workers (see ZIEGLER, 1984; SCHAFFT, 1993) and whose effects take place over a long period of time.

6.5 Limitations related to experimental conditions

The relevance of the results of the experiments for tropical conditions is only possible if limitations of experimentation are considered. Crossbreeding Cameroon hair sheep with German mutton and milk breeds to produce C1 and C2 in Berlin and the assessment of their productivity is an important initial step in any effort to promote rearing in the Tropics. However, station research conducted in Dahlem, Berlin, is of limited application where no follow up field research work is done under tropical conditions.

Confinement was necessary to obtain exact data especially relating to feed and water consumption. Confinement, however, excludes the complex effects of direct (radiation, conduction and convection, wind, precipitation and atmospheric pressure) and indirect climatic factors as well as the natural behaviour of animals under variable conditions of climate.

The danger of eroding local genetic resources if no deliberate attempt is made by governments, organisations and individuals concerned, to conserve them - becomes imminent. But considering the drought prone parts of the Tropics where proceeds from the crop production sector are used to purchase more animals whose sale increases in drought years and market peaks during the year, increasing the efficiency of production by rearing crossbred animals produced by mating between exotic animals selected for high performance and local animals adapted to the local environment, is inherently a means to conserve local animal genetic resources.


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