It is well established within the swine industry, that
when temperatures exceed 29 8C (85 8F) with 85%
humidity for more than 10 d, spermatogenesis is
negatively impacted (Burke, 2007; personal communication).
In the area where most of our boar studs are
located, summer temperature and humidity often
exceed these levels. Northern European breeds of
swine have lower tolerance for heat stress than breeds
developed along the equator; differences are clearly
evident among genetic lines [1]. Differences in
percentage of ejaculates discarded (trashed) for four
genetic lines in boar studs in the US, as well as their
trash rates during seasonal infertility, are shown
(Table 1).In our system, ejaculate trash rates consistently
followed a breed trend over a large number of ejaculates
(Table 2). During the more thermo-neutral or cool
months (November to June), trash rates were the highest
(poorest semen quality) in the purebred maternal lines,
with the Yorkshire having approximately a 2% higher
trash rate than the Landrace. Next was the purebred
terminal Duroc breed. The lowest trash rates (best
semen quality) were in the crossbred terminal lines,
with the Duroc–Hampshire F1 crossbred having the
lowest trash rate, followed by the 3/4 Duroc–1/4
Hampshire crossbred. The breeds ranked similarly in
their sensitivity to warmer temperatures (July to
October), and seasonal infertility. The maternal purebred
lines were the most sensitive to seasonal infertility.
During the warmer season, their trash rates were very
similar. Next was purebred Duroc, which produced
approximately 5% higher trash rates during the warmer
season, but at levels consistently better than the
maternal lines. Lastly, the crossbred terminal lines
encountered a moderate increase in trash rates during
the warmer months.