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°C
for farrowing rate, 21.7°C for litter size, and 19.6°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.