ASSOCIATIONS BETWEEN LITTER SIZE AND SPECIFIC SOW-LEVEL MANAGEMENT FACTORS IN ONTARIO SWINE

This retrospective study examined associations between sow-level manag ement factors and litter size in a target population of Ontario swine farms that used computerized recording systems between 1987 and 1991. The data were from 112 herds with 66 525 individual sow records. The a verage litter sizes were ten pigs (SD = 2.7) for primiparous sows and 11.4 pigs (SD = 3.1) for multiparous sows, respectively. The total bor n litter size (including pigs born alive, stillborn and mummified) was regressed on the sow-level variables of interest, Multiple regression models were built using a backward elimination approach and variables significant at P<0.05 were retained. There was a curvilinear relation ship between litter size and parity; the largest litters were from sow s in parities three through ten. Natural matings resulted in one more pig per litter than artificial insemination. Lactation lengths of 27-3 2 days and 33-40 days resulted in progressively larger litter sizes in subsequent farrowings than shorter lactation lengths. Litter size was highest at weaning-to-concepti on intervals of up to 4 days, decrease d daily from 5 to 7 days, at which point it reached a plateau, until D ay 11 when it increased again. The sow's current litter size was posit ively associated with her previous litter size. The model explained on ly 18% of the variation in litter size. As the age of the gilt at firs t conception increased, the litter size increased.