- Description of growth and feed efficiency of Dorper Lambs
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The description of growth and feed efficiency of Dorper Lambs
Grootfontein College of Agriculture, Middelburg C.P., 5900
Breed and sex difference in voluntary feed intake of a specific diet, difference in body composition and maintenance requirements at a fixed age or mass exist among sheep (Blaxter, 1964 as quoted by Kleiber, 1969; Rogerson, Ledger & Freeman, 1968; Meissner, Roux & Hofmeyr, 1975; Meissner, 1983). Taylor (1980) pointed out that animals with a larger frame size will generally take longer and consume more food to reach the same stage of maturity in body mass than their smaller counterparts. Available results indicate that the difference in composition of growth can also be connected to frame size (Taylor, 1980; Meissner & Roux, 1983) and that these differences tend to decline as growth proceeds, which suggests that the differences in energy requirements for growth also decline towards maturity. Thus, effective predictions of energy requirements for growth are dependent on knowledge of the total growth curve, a field which has not been studied for the Dorper Sheep.
Compositional changes affect efficiency and consequently it may be expected that the same variables which influence the composition of gain could have a parallel effect on the efficiency with which gain is realized. In sheep the efficiency of utilization of energy for gain, as with the composition of growth is dominated by production function and climatic adaptation (Hofmeyr, 1972; Meissner, 1983). The need to accumulate knowledge on the efficiency of feed utilization is imperative for the calculation of energy requirements.
In this study all the Iambs were considered as an input-output system (Roux, 1976) and consequently the pattern of food intake was described and related to live mass in order to estimate mature size and efficiency of growth. All the results were scaled by percentage of mature mass in order to approximate "physiological age". By doing this, comparisons within growth phases decrease interaction effects substantially.
A total of 78 Dorper Iambs, comprising 26 ewes, 26 rams and 26 wethers were used. These Iambs were removed from their mothers at one day of age, their body masses recorded and individually penned. Ram Iambs scheduled to form the wether group were castrated by means of elastic rings within 24 hours after birth. All Iambs were hand reared on a milk substitute, administered on an ad libitum basis, and discontinued after 50 days.
All Iambs had free access to a pelleted diet from 2 weeks of age until the end of the experiment. The diet consisted of 50% lucerne hay, 38% maize meal, 10% fish meal, 1,0% Ca (HPO4)2, 0,5% CaCO3 and 0,5% NaCL, with a ME content of 10,19 ± 0,442 MJ/Kg and a crude protein content of 19,6 ± 0,66%.
One Iamb per sex group, was sequentially slaughtered, starting at 1 week of age with weekly intervals til113 weeks of age and thereafter fortnightly till the age of 39 weeks. Slaughtering was carried out after deprivation of food and water for 18 hours. In addition to normal slaughtering procedures care was taken that all the blood was collected. The methods in mincing, sampling and chemically analysing the carcass and the offal components are described by Hofmeyr (1972).
RESULTS AND DISCUSSION
It is generally accepted that large-frame types of animals have a higher basal energy expenditure than smaller framed types if size is corrected for through metabolic mass (Wkg 0.75) (Anderson, 1978). On the other hand Meissner, Van Staden & Pretorius (1982) have pointed out that corrections made through metabolic mass do not correct to the same physiological age as does a particular percentage of mature mass. In this study the growth-model as described by Roux (1974, 1976) was used and the percentage of the exponent aw, expressed as mature mass, was used as basis of comparison. The growth interval taken into account was between 30 and 80% of mature mass.
From Table 1 it is clear that the mature mass of rams was superior to that of ewes and wethers. The ratio between mature mass of rams and the mature mass of ewes and wethers was 1,25 and 1,00 respectively. Although these ratios varied between sexes as expected they seem to disagree with the ratio of 1,30 proposed by Hammond (1932). Thompson & Parks (1983) reported values of 1,23 and 1,28 for different strains of Merino rams, while Blaxter, Fowler & Gill (1982) found a clear difference in the mature mass of rams and wethers. These ratios are of basic importance to understand the difference between sexes and their feeding, growth and body composition. In general the patterns of the food-intake curves were similar for the three sexes (Fig. 1). The curves increased to a maximum intake and then decreased as mature size is approached. These results were in accordance to the equation of diminishing return reported by Blaxter, et al (1982).
Rams obtained the highest intake level and reached their maximum level at 60% of maturity. The intake of ewes was the lowest of the three sexed but also reached their maximum at 60% of maturity, while the intake level of wethers was intermediate to those of rams and ewes after 50% of maturity. Wethers reached their optimum intake at 50% of maturity.
The gain in protein of rams was superior to that of ewes and wethers, while the differences in gain between ewes and wethers were small (Fig. 2).
According to the gain in fat deposition between the sexes (Fig. 3), it was evident that the fat gain in wethers accelerated at a much faster rate than in ewes or rams. The highest gain of 142,1 g/d for wethers occurred at 60% of maturity, while in ewes the rate was 98,7 g/d at 60% maturity. The rate of ewes was the lowest and that of rams intermediate between ewes and wethers.
From the metabolizable energy intake and the energy stored in body tissues, the efficiency (ME intake/energy retention) can be calculated. According to Fig. 4 it is clear that the efficiency of rams decreased with an increase in maturity, while that of ewes and wethers increased with maturity. The efficiency of wethers was significantly better in the later stages than that of rams and ewes.
A possible explanation for the differences in efficiency could be found in the differences in the amount of energy lost (Fig. 5). Part of this energy loss is termed maintenance expenditure. The energy loss of rams increased and therefore resulted in a decrease in efficiency. In the case of wethers exactly the opposite efficiency was obtained. The energy loss of ewes also decreased but not to the same extent as in wethers. Graham, Searle & Griffiths (1974) reported a decline in basal metabolic rate with age that was independent of the growth rate effect and that this may contribute to the decline. On the other hand, Roux, Meissner & Hofmeyr (1982) found that the efficiency of protein deposition decreases with an increase in body mass (increase in maturity) and that of fat increases with body mass. With the latter in mind as well as the fact that fat gain as a percentage of total gain of wethers increased rapidly with maturity (Table 1), it seems clear why the energy loss of wethers decreased. This explanation is also applicable to the energy loss of ewes but to a lesser extent. As far as the energy loss of rams is concerned, it appears that rams have a higher maintenance requirement at the same proportion of mature size than ewes and wethers. Thompson & Parks (1985) also reported that, after scaling for differences in mature: size, rams had a greater rate of energy loss than ewes from weaning to maturity. Webster (1981) reported that lean sheep had a greater heat production than fat sheep, when adjusted by body mass raised to the power 0,75, because protein is the most metabolically active tissue in the body and contributes most to heat production. Thompson & Parks (1985) also postulated that the differences in energy lost between sexes were to a large degree related to the differences in protein content in the body. From Table 1 it is clear that rams have the highest protein content of all the sexes and that the results of this study were in accordance with those of Thompson & Parks (1985).
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