MODIFICATION OF BODY-COMPOSITION BY ALTERING THE DIETARY LYSINE TO ENERGY RATIO DURING REARING AND THE EFFECT ON REPRODUCTIVE-PERFORMANCE OF GILTS

Fifty-four lean genotype crossbred gilts were allocated at 118 days of age among three diets with different protein concentrations to give l ysine : energy (g/MJ digestible energy) ratios: high (0.9), medium (0. 6) and low (0.3) given twice daily at 2.9 X maintenance energy. At 160 days of age, gilts were treated with exogenous gonadotropin (PG600(TM )) and animals were examined daily for signs of oestrus. Animals were slaughtered after the second oestrus, if they had shown behavioural oe strus, or at the age of 212 days. Reproductive tracts were recovered f or counting of corpora lutea and albicantia. L gilts were lighter than M or H gilts at puberty induction (80, 95, 97 (s.e. 0.73) kg for L, M , H respectively, P < 0.001), with greater backfat thickness (10.8, 10 .0, 9.2 (s.e. 0.21) mm P-2, P < 0.001), lesser longissimus muscle dept h (57.4, 65.9, 64.3 (s.e. 0.77) mm, P < 0.001) and poorer food convers ion ratio during rearing (3.87, 2.48, 2.42 (s.e. 0.098) kg food per kg gain, P < 0.001). There was no statistically significant difference i n the total number of animals that responded to the puberty induction, although L had a greater latency to oestrus than H. Ovulation rate at this induced oestrus was significantly lower in L gilts than in M or H gilts (12.5, 17.3, 21.5 (s.e. 1.32), P < 0.01). A lower proportion o f L, compared with M or H gilts showed spontaneous ovulation in a subs equent cycle (0.15, 0.75, 0.77, chi(2) = 12.72, P < 0.005). It proved possible, by means of low protein (lysine) diets, to increase body fat reserves in breeding gilts but protein restriction in the rearing pha se negatively affected aspects of reproductive performance.