Accounting for lactation length and weaning-to-conception interval in genetic evaluations for litter size in swine

Effects of lactation length and weaningto-conception interval on the subseq uent Litter size of purebred sows were estimated using an animal model. Dat a on 2,847 Landrace sows with 7,125 litters born between January 1989 and M ay 1997 and on 1,234 Yorkshire sows with 2,999 litters born between January 1990 and May 1997 were obtained from two Canadian selection herds. Sows ha ving a lactation of less than 14 d (MMEW) were usually not mated until thei r second estrus, whereas sows weaned after at least 14 d of lactation (late r weaning) were usually mated on their first estrus. Litter size included b oth number of pigs born alive and those stillborn. Linear, quadratic, and l ogarithmic effects of lactation length were tested. The effect of weaning-t o-conception interval on litter size was modeled using an approach based on threshold variables and an approach using segmented polynomials. Results i ndicated linear and logarithmic effects of lactation length on subsequent l itter size for Yorkshire and Landrace breeds, respectively. Litter size dec reased as weaning-to-conception interval increased up to 7 and 10 d for Yor kshire and Landrace, respectively, then increased with further increases in weaning-to-conception interval up to 35 and 30 d for the two breeds, and t hen remained constant. The MMEW sows did not have lower subsequent litter s izes than later-weaned sows because the negative effect of a shorter lactat ion was offset by the positive effect of a longer weaning-to-conception int erval. However, average time spent open per parity was longer for MMEW sows than for later-weaned SOWS. Both lactation length and weaning-to-conceptio n interval should be considered in models for the genetic evaluation of lit ter size in purebred swine. Segmented polynomials can be used to predict li tter size as a continuous function of weaning-to-conception interval or to derive weaning-to-conception interval adjustment factors for litter size.