Item 1, Topic 1
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4.1.3 Sex effects

Sex effects on body composition can be viewed using the same concepts of mature weight, mature composition and maturing patterns for individual tissues.

Entire males vs females

Rams and bulls are generally 1.4 times the mature weight of females (i.e. 40% greater).  As shown in Table 4.3, mature composition between sexes differs in that males have less fat, more muscle and more bone than females.

Table 4.3 Percentage body composition in mature rams and ewes (Thompson et al. 1984).

In contrast to between breed differences, entire males and females have differences in composition at maturity and there are differences in maturity patterns for the tissues relative to bodyweight (Figure 4.9).

Figure 4.9 Maturing patterns for muscle and fat relative to live weight in rams and ewes.

The difference in mature composition and maturing pattern for the various tissues means that the relative composition of rams and ewes changes depending upon the live weight, or stage of maturity at which the comparison is made (Figure 4.10).

Figure 4.10 Changes in the percentage of dissectible fat in the body of Merino rams and ewes (a) relative to live weight, and (b) stage of maturity during progress to maturity. Changes in the percentage of muscle in the body of Merino rams and ewes (c) relative to live weight, and (d) stage of maturity during progress to maturity (Thompson 1983).

Entire males vs castrates

Castration of entire males is commonly undertaken in Australia, Britain and North and South America.  This contrasts to much of Europe where entire males are often used for meat production.  Historical reasons for castration include the increased fattening ability of the castrate.  In addition it was often used as a management aid and allowed for controlled breeding.

Table 4.4 Mature body composition of rams and wethers (Butterfield et al. 1985)

Table 4.4 shows that after adjustment for the increased weight of the head and horns and testes in the rams, rams and wethers had a similar mature weight, although there were large differences in mature composition.  At maturity rams had less fat and more bone and muscle than wethers.  Also there was no difference in maturing patterns for muscle, bone and fat.  Because there was no difference in mature size, comparisons were similar whether they were made at the same stage of maturity, or body weight.  Rams were leaner than castrates at all stages, with the magnitude of the difference increasing with increased live weight (or stage of maturity).