- Effect of compensatory growth on feed intake composition and efficiency of feed utilization in Dorper sheep
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THE EFFECT OF COMPENSATORY GROWTH ON FEED INTAKE, GROWTH RATE, BODY COMPOSITION AND EFFICIENCY OF FEED UTILIZATION IN DORPER SHEEP
Grootfontein Agricultural College, Middelburg Cape Province, 5900 Republic of South Africa
Compensatory growth is manifested in the ability of animals previously restricted in feed or nutrient intake to outgain their better counterparts when given free access to good quality feed. The effect of compensatory growth in animals has been reviewed by Wilson & Osbourn (1960), Allden (1970) and O'Donovan (1984). According to these reviews, results that were obtained were contradictory. These contradictions may possibly arise from differences in body composition, voluntary feed intake, the effect of age and efficiency of feed utilization.
The effect of compensatory growth on body composition in sheep has been studied by various researchers. Some experiments have shown that there are no differences in body composition between continuously grown and realimented animals (Kellaway, 1973; Searle & Graham, 1975; Thornton, Hood, Jones & Re, 1979). Other researchers have shown that realimented sheep are leaner than continuously grown animals (Burton, Anderson & Reid, 1974; Drew & Reid, 1975), whilst others have shown that realimented animals contain more fat (Ledin, 1983; Notter, Ferrell & Field, 1983). In all these experiments, mature animals were used as experimental material. According to Searle, Graham & Smith (1979), immature sheep are even more vulnerable to undernutrition, particularly in the period immediately after weaning. The end results will thus depend on the physiological age and the time that treatment started (Gunn, 1964a,b). Further contributing factors could include different restriction levels, different periods of restriction and realimentation, different protein levels as well as breed (Hofmeyr, 1972; Meissner, 1977) and sex (Meissner, 1977; Marais, 1984) differences.
According to Thompson, Bickel & Schurch (1982), compensatory growth can be explained in terms of an increased efficiency of feed utilization. Anderson (1975) reported that most experiments with cattle indicate that feed conversion on restricted feeding is more efficient than with ad libitum feeding. Meissner, Hofmeyr & Roux (1977) reported the same effect in sheep and detected a change in composition of growth, which suggests that more protein but less fat was deposited.
Higher feed intake after a period of feed restriction has been reported in the majority of experiments (Wilson & Osbourn, 1960; Graham & Searle, 1975; Thornton, et al., 1979; Greeff, 1984). Contrary to these results, Drew & Reid (1975), Murray & Slezaceck (1980) and Hogg & Tulloh (1982) reported that feed intake did not increase.
Owing to the fact that body composition, voluntary feed intake and maintenance requirements differ between breeds, it is possible that Dorpers may react differently to feed restrictions. Knowledge of the effects of feed restriction on growth rate and efficiency of feed utilization is important for economic reasons.
This study was conducted to quantify the effects of various feeding levels on feed intake, growth rate, body composition and efficiency of feed utilization during restriction and realimentation on Dorper sheep.
MATERIAL AND METHODS
A total of 48 Dorper Iambs, comprising 24 ram and 24 ewe Iambs, obtained from 100 Dorper ewes from the Grootfontein Agricultural College stud were used. Only single-born male and female Iambs were included in the experiment. Owing to the fact that a clear break in the relationship between 1n(cumulative ME intake) and 1n(body mass) occurs at 13 weeks of age (Meissner, 1977; Marais, 1984), care was taken to select animals older than 13 weeks of age. The Iambs were subdivided into 4 groups of 6 ewes and 6 rams each. The mean body mass at the commencement of the trial was 24,3 ± 5,1 kg. The groups were allocated to four different feeding levels as follows: ad libitum, and 80%; 65% and 50% of ad libitum Lambs were housed in individual pens from about 2 weeks prior to the commencement of the experiment, until the body mass of ewe Iambs reached 45 kg and that of ram Iambs 55 kg. Animals were fed individually and the allocated amount of feed of each Iamb was weighed out at the beginning of each week. Daily amounts of feed were given to restricted groups in two equal portions. Drinking water was freely available.
From the beginning of the experiment, the Iambs received restricted feeding, except for the control group on ad libitum intake, for 9 weeks. After this period, the restricted groups received ad libitum feeding. It was thus necessary to calculate the amount of feed of the restricted groups on a weekly basis.
Individual intakes and live masses were determined weekly. Live mass was determined at 08:00 without prior fasting. Although this procedure is recognised as being less reliable owing to differential gut-fill, a period of fasting could interfere with the measurements of "true" ad libitum intake. By fitting a mathematical function to the live mass data measurement error is reduced, which would render this procedure acceptable. Body composition was estimated at 2 to 3 week intervals by the tritium dilution method (Meissner & Bieler, 1975). The cumulative ME intake of individual Iambs prior to the commencement of the trial was calculated from the linear regression equation between 1n (cumulative ME intake) and 1 n (body mass) as described by Meissner (1977).
Composition of diet
The ingredients used to compile the pelleted diet are shown in Table 1.
Table 1. Composition of the diet on an air dry basis.
ME- content 10,15 ± 0,23 MJ/kg
Crude protein 15,44 ± 0,14%
Digestibility of the diet
As Iambs were fed at different intake levels, differences in digestibility of the diet were expected. Thus the digestibility of the diet was determined in vivo. lambs were distributed at random between weeks of faeces collections, with each Iamb completing at least three periods of collection before the end of the trial. The metabolisable energy (ME) intake of each Iamb was computed from the digestible energy (DE) intake multiplied by 0,82 (Blaxter, 1962).
RESULTS AND DISCUSSIONS
Apparent dig estibility of the diet.
The apparent digestibility of the diet is illustrated in Table 2.
Table 2. Energy digestibility (DE) of the diet of the different treatments during the restriction and realimentation phases.
|Restriction phase||Realimentation phase|
|Treatment||X ± SD||X ± SD|
|Ad lib.||68,84 ± 3,28||68,84 ± 3,28|
|80%||69,05 ± 2,51||68,74 ± 2,81|
|65%||71,02* ± 2,36||68,45 ± 2,43|
|50%||73,07* ± 2,24||68,53 ± 2,23|
* Differ significantly (P<0,05) from ad lib. value.
The values for digestibility obtained from each Iamb were pooled and subjected to variance analysis. According to Table 2, digestibility increased (P<0,05) as feeding level decreased during the restriction phase, probably as a result of the longer retention time in the rumen. This also indicates that Iambs of different groups consumed different amounts of digestible energy. During the realimentation phase, no significant differences in digestibility of the diets were found between treatments.
The energy intake (MJ ME/day) during the restriction and realimentation phase of different treatments, at different body masses, as calculated from the allometric autoregression model, is illustrated in Figure 1 and 2. Energy intake between sexes at the same live mass differed considerably during the restriction phase. If the end of the restriction phase is taken as a reference point, then the energy intake of ewe Iambs was 17,1; 13,7; 11,3; and 7,6 while that of ram Iambs was 21,7; 15,2; 13,6 and 11,3 respectively for ad libitum, and 80%, 65% and 50% of ad libitum.
With the exception of the energy intake of the ewe Iambs on the 80% ad libitum intake, the intake of all the other Iambs in the restriction groups increased at the beginning of the realimentation phase. The increase in feed intake was 1,5%, 28,3% and 92,1% for the 80%, 65% and 50% intake groups respectively. Only the feed intake of the 50% group exceeded the intake of the ad libitum group at the 28 and 30 kg live mass range. After the rise in feed intake, the intake of the 65% and 50% ad libitum groups decreased with an increase in live mass.
An increase in the ME- intake of ram Iambs was also observed at the beginning of the realimentation phase. The increase in feed intake was found to be 16,5%, 34,6% and 48,7% for the 80%, 65% and 50% intake groups respectively. With the exception of the intake from the group receiving 50% of ad libitum, the rest of the restriction groups recorded intakes that exceeded that of the ad libitum group. These results agree with the general findings of Graham & Searle (1975), Saudibet & Verde (1976), Thorton, Hood, Jones & Re (1979) and Greeff (1984) that feed intake increases after a period of feed restriction. In spite of the increase in energy intake of ewe and ram Iambs that occurred at the beginning of the realimentation phases, only the intake of ewe Iambs on the 50% intake between 28 and 30 kg and that of ram Iambs on the 80% and 65% intake could better the intake of the ad libitum group at a specific live mass.
Figure 3 and 4 illustrates the growth rates (g/day) of the various treatments during the restriction and realimentation phases as calculated from the allometric autoregression model for ewe and ram Iambs. As the restriction increased, growth rate decreased. If the end of the restriction phase is taken as reference point, then the growth rate of ewe Iambs was 231; 201; 153; and 70 while that of ram Iambs was 276; 222; 173 and 128 (g/d) respectively for intake levels of ad libitum, 80%, 65% and 50% of ad libitum intake. At the start of the realimentation phase there was in some cases a remarkable increase in growth rate. The growth rate of ewe Iambs increased by 8,9%, 45,8% and 251,4% for the 80%, 65% and 50% intake groups respectively. In spite of these increases in growth rate, the growth rate of the restricted groups could not equal or better the rate of the ad libitum group at a specific live mass.
At the start of the realimentation phase, the growth rate (g/d) of ram Iambs increased by 104,5%; 104,6% and 174,2% for the 80%, 65% and 50% intake groups respectively. Owing to these increases the growth rates of all the restricted groups better the rate of the ad libitum group at a specific live mass.
Thus, it seems clear that superior gains were generally observed following restrictions. Higher growth rates can be attributed to extra water in the gut (Keenan, McManus & Freer, 1969; Drew & Reid, 1975). Increased appetite and its associated gut-fill effects could also be important contributory factors responsible for compensatory growth especially at the beginning of the realimentation phase.
The total amount of protein, as well as their percentage at different body masses, are indicated in Table 3 for the restriction and realimentation phases.
No differences in the total amount of protein within sex groups at the same body mass could be detected between feeding levels during the restriction phase. With the exception of the amount of protein which showed a tendency to decrease in the bodies of ram Iambs on an intake of 50% ad libitum, no differences between feeding levels could be detected during the realimentation phase. These results confirm the findings of Searle, Graham & Donnelly (1982) that feeding levels had no significant influence on the amount of protein at a specific body mass. The general pattern of these results supports the findings of Elliot & O'Donovan (1969) and Meissner & Hofmeyr (1976) that the protein content of animals is remarkably constant at a particular body mass.
The deposition rates of protein at different body masses are illustrated in Figure 5 and 6 for the restriction and realimentation phases. It is clear that the deposition rate of protein decreased progressively as restriction during the restriction phase increased. With the onset of the realimentation phase a large increase in deposition rate of both sexes took place. The percentage increase for ewe Iambs was 244,4% for the 50% intake, 59,1% for the 65% and 5,4% for the 80% intake group. The increase for ram Iambs at the same feed intake was 190%, 121,9% and 118,0% respectively. In spite of the drastic increase in deposition rate of ewe Iambs at the beginning of the realimentation phase, the rate of growth in the groups that were restricted could not equal or better the rate of the ad libitum group. However, in the case of the ram Iambs, the deposition rate of the 80% and 65% intake groups exceed that of the ad libitum group. These results confirm data reported by Reid, Bensadoun, Burton, Gleeson, Han, Joo, Johnson, McManus, Paladines, Stroud, Tyrrell, van Niekerk & Wellington (1968); Keenan, McManus & Freer (1969); Thompson, Bickel & Schürch (1982) and Greeff, Meissner, Roux & Janse van Rensburg (1986) showed that protein deposition increased after a period of undernutrition. In general, sex differences in the deposition rate do occur during the restriction and realimentation phases.
The total amount of fat in the carcass, as well as their percentage at different body masses, is given in Table 4 for the restriction and realimentation phases. No differences in the total amount of fat, within sex groups at specific body masses, could be detected between different feeding levels. The total amount of fat in ewe Iambs, at a specific body mass, was higher than that of ram Iambs.
The deposition rate of fat progressively declined (Figure 7 and 8) with an increasing restriction. The magnitude of the increase during the beginning of the realimentation phase was not large enough to better the rate of the ad libitum group. During the realimentation phase, the deposition rate of fat in ram Iambs increased at the onset of this phase to such an extent that the rate of all the restricted groups bettered that of the ad libitum group.
Fat: Protein ratio
The proportion of fat to protein deposited at different body masses for the restriction and realimentation phases is illustrated in Figure 9 and 10. It thus seems clear that feeding levels did not affect the fat: protein ratio of ewe and ram Iambs. As expected, the ratio of ewe Iambs was greater than that of ram Iambs. There was a slight increase in the fat: protein ratio of ram Iambs on the 50% ad libitum intake group during the realimentation phase. A reason for this slight increase could be found in the lower deposition rate of protein in this group.
Efficiency of energy conversion
The efficiency of energy conversion (MJ ME intake/MJ retained) that was measured during the restricted and realimentation phases is illustrated in Figure 11 and 12 for ewe and ram Iambs. It is clear that as the restriction increased, efficiency of energy conversion declined. These results were in close agreement to the results of Greeff (1984) and Greeff, Meissner, Roux & Janse van Rensburg (1986) but conflicted with those of Meissner, Hofmeyr & Roux (1977). The efficiency of ram Iambs was constantly better than that of ewe Iambs throughout the restriction phase. During the realimentation phase, efficiency of energy conversion improved in all cases which showed an increase in live mass. The efficiency increased proportionally to the previously imposed restriction. In spite of these increases, ewe Iambs from the restricted groups could not equal or better the utilization of the ad libitum group at a specific live mass. Ram Iambs follow the same utilization pattern, with the difference that the restricted groups tended to improve on the efficiency of the ad libitum group from 46 kg live mass upwards. These results supported the findings of Greeff (1984) and Greeff, Meissner, Roux & Janse van Rensburg (1986) but contradicted the results of Meyer & Clawson (1964), Allden (1968) and Jacobs (1972) who found that gross efficiency of energy conversion is not influenced during realimentation.
An important question that arises lies in whether efficiency over the entire trial period was better for certain groups than others. Table 5 gives the feed efficiency for the entire period. According to Table 5, the efficiency of ewe Iambs over the entire trial decreased with increased restriction of feed intake, while that of ram Iambs, except the value of the 65% intake, showed an improvement in efficiency of energy conversion.
The results from the ewe Iambs do not agree with the general trend described by Meissner (1983), i.e. that the efficiency of energy conversion may improve above that of ad libitum intake when feed intake lies between ad libitum and 70% of ad libitum intake. Results of this study agree with the findings of Wilson & Osbourn (1960) and suggest that the advantage of an increased efficiency during realimentation may be completely eliminated by the reduced efficiency during the restriction phase.
The results indicate that when the total amount of protein or fat or fat: protein ratio is taken as the criterion of body composition, when examining compensatory growth in Dorper sheep, no significant differences between feeding levels were found. This confirms the findings of Tulloh, (1963); Reid, et al, (1968); O'Donovan, (1974) and Basson, (1975) that body composition is uniform at a specific body mass and is independent of feeding level. On the other hand, when deposition rates of protein or fat together with an increase in body mass are taken as criteria, compensatory growth is only exhibited by ram Iambs after a period of undernutrition. It seems that sex differences were present after a period of feed restriction. This indicates that restriction of ewe Iambs would inhibit production and that it would not be economically feasible to do so in order to obtain an increased efficiency of energy conversion. In the case of ram Iambs, feed restrictions of up to 50% for 9 weeks resulted in a better energy conversion compared to ad libitum feeding for the whole growth period.
Dorper News No 51, 1992