Gaughan et al. (1995, 1997) have described the effects on reproductive maturation of long-term genetic selection for lean. But, the extent to which nutrition directly affects the onset of puberty remains controversial. Some work appears to indicate that there are minimum thresholds for body weight and body composition that must be achieved prior to the onset of puberty (Dickerson et al., 1964; Kirkwood and Aherne, 1985), whereas other work suggests that the gilt must reach a certain age before the advent of puberty (Burnett et al., 1988; Rozeboom et al., 1995).
It is relatively simple to change the fat content of breeding pigs through manipulation of daily energy intake. More complex is the relationship between energy and amino acid intake and the consequent effects on both lean and fat growth. Cia et al. (1998) fed diets to the gilts with lysine/energy ratios of 0.9, 0.6 and 0.3 g lysine per MJ DE to test a hypothesis that modification of body composition which favours fat deposition in lean-type gilts would encourage early puberty. However, restricting dietary amino acids to increase body fat content did not improve reproductive performance of lean-type gilts but delayed puberty. Jones and Maxwell (1974) and den Hartog and Verstegen (1990) also found that amino acid restriction during the prepubertal period did not advance the appearance of puberty. Others have reported that providing a low-protein diet during the rearing period depresses growth rate and delays puberty (Wahlstrom and Libal, 1977; Cunningham et al., 1974). Mild amino acid restriction, however, does not appear to influence the age at puberty (den Hartog and van Kempen, 1980).
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