EFFECT OF DELAYED BREEDING ON THE ENDOCRINOLOGY AND FECUNDITY OF SOWS

The effect of serving sows at first or second estrus after weaning was determined using 114 Camborough sows (Parities 1 to 8). Eighty sows were paired according to weaning-to-estrus interval and parity. One sow of each pair was bred at first and the other at second estrus after weaning. A further 27 sows were catheterized on d 3 after weaning or d 20 of the first estrous cycle and bred at first or second estrus, respectively. Blood samples were taken every 30 min from 0800 to 1330 on the day after catheterization, every 6 h from catheterization until 1 d after standing estrus, and every 12 h for a further 3 d. Of the 80 sows, litter size increased (P < .002) by breeding Parity 1 and 2 sows at second rather than at first estrus (10.4 vs 12.8 total pigs born). These increases in litter size were achieved in the absence of any significant changes in sow weight or backfat during lactation and between weaning and breeding, and predicted changes in fat and protein during lactation. Similar but nonsignificant results (P = .15) were observed for the 27 catheterized sows. Plasma progesterone concentrations measured 50 h after the preovulatory LH surge were higher in Parity 1 and 2 sows bred at second rather than at first estrus (5.4 +/- .7 vs 3.7 +/- .3 ng/mL, P < .04). Sows of Parity 1 and 2 bred at first rather than at second estrus had higher plasma IGF-I(P < .05) and glycerol (P < .001) concentrations and tended to have lower preprandial insulin concentrations (P = .09). Among sows of Parity 3 or more, no significant differences were seen in plasma insulin, IGF-I, and glycerol concentrations between first and second estrus, but growth hormone was lower at second estrus (P < .03). Delaying the breeding of Parity 1 and 2 sows increased subsequent Litter size, possibly by increasing embryo survival. Furthermore, insulin, IGF-I, and glycerol, but not growth hormone, seem to be useful measures of metabolic status and potentially play a role in regulating sow fertility after weaning.