Factors that influence litter size in swine: parity 3 through 7 females.

The influence of 7 factors on litter size for the combined parities 3 through 7 (age at conception of the first litter, parity-1 litter size, duration of lactation in the preceding parity, weaning-to-conception interval in the preceding parity, farrowing-to-conception interval in the preceding parity, number of matings per conception, and month of conception) was investigated in 11,929 litters from 5 commercial herds. Age at conception of the first litter did not influence litter size in these parities in any herd. In the 4 herds that kept such data, duration of lactation negatively influenced the weaning-to-conception interval, and duration of lactation and weaning-to-conception interval positively influenced the litter size in the next parity. Because the effects of duration of lactation and weaning-to-conception interval could not be investigated separately, their effects on litter size were simultaneously investigated, using the farrowing-to-conception interval for these combined parities. In all herds except one, the increase in live litter size was between 0.02 and 0.09 pig for each day increase in the preceding farrowing-to-conception interval less than 36 days (P less than 0.05). Duration of lactation within this farrowing-to-conception interval similarly influenced litter size. Litter size in litters of any herd was not affected by a preceding farrowing-to-conception interval greater than 35 days. In all herds except one, mean litter size of females with a farrowing-to-conception interval greater than 35 days was 0.26 to 0.96 pig greater (P less than 0.1) than for females with a farrowing-to-conception interval greater than 36 days. In 1 of 3 herds that kept such data, 2 matings per conception resulted in a larger mean live litter size than 1 or 3 matings (P less than 0.05). When these females were grouped on the basis of their conception during one of the 3-month seasonal periods, no influence of season on litter size was found. The effect of parity-1 litter size on litter size of subsequent parities 3 through 7 was significant. The regression coefficient ranged from 0.11 to 0.22 (P less than 0.01) in the farrowing-to-conception interval less than 36 days in all herds. In females with a farrowing-to-conception interval greater than 35 days, this association was significant only in one herd.(ABSTRACT TRUNCATED AT 400 WORDS)