Sow line differences in heat stress tolerance expressed in reproductive performance traits.

The objectives of this study were 1) to investigate if there were differences in the relation between temperature and reproductive performance traits in 2 different sow lines, a Yorkshire line producing mainly in temperate climates and a Large White line producing mainly in warm climates, and 2) to determine the upper critical temperature (UCT) for the reproductive performance of these 2 lines. Sows are exposed to heat stress when temperature exceeds the UCT of the thermo-neutral zone. Data included 32,631 observations on reproductive performance from 11,935 sows on 20 farms in Spain, collected from 2003 to 2005. Sows belonged to 2 different purebred sow lines, named D (Yorkshire sow line, producing mainly in temperate climates) and I (Large White sow line, producing mainly in warm climates). Only first insemination records per parity were used and were combined with the maximum outside temperature at day of insemination. Upper critical temperatures were studied for 3 reproduction traits: farrowing rate (0 or 1), litter size (range from 1 to 25), and total number of piglets born per first insemination (combination of farrowing rate and litter size, range from 0 to 25). Data were corrected for fixed effects, which included parity, service sire, and an interaction between farm and year. Corrected data were used as observations in the models to study the effect of outside temperature on reproductive performance. Two models were compared for goodness of fit: a linear regression model and a plateau-linear model with the plateau representing the thermo-neutral zone and a linear decrease above that zone. Farrowing rate of I-line sows was not affected by temperature. For litter size and total number born per first insemination of I-line sows no UCT could be estimated. These traits were linearly affected by temperature. For all 3 reproduction traits of the D-line the best model was the plateau-linear model; the UCT for the D-line sows was estimated to be 19.2 degrees C for farrowing rate, 21.7 degrees C for litter size, and 19.6 degrees C for total number born per first insemination. The decrease in reproductive performance of I-line sows with increasing outside temperature was less than in D-line sows. From this study it can be concluded that there are differences in heat stress tolerance between sow lines as measured by the differences in reproductive performance. These differences are an indication of genetic differences in heat stress tolerance in sow lines.