Heat stress is well known to have negative effects on reproduction, such as delayed puberty and irregular returns to estrus in gilts. Yet limited data is available to describe estrous traits under seasonally warm conditions for modern genetic lines.

The objective of this study was to illustrate gilt puberty characteristics during warm summer temperatures for a popular genetic line. A cohort of 393 Landrace x Large White composite gilts were reared at the North Carolina Department of Agriculture (NCDA) Tidewater Research Station, Plymouth, NC. Groups of 15 gilts were placed in modern, curtain-sided buildings on fully slotted floors (9 sq. ft./gilt). Fans and timed misters were used for cooling when temperatures reached 80°F.

Starting at 130 days of age, each group of gilts was penned with three mature boars for seven minutes daily and estrous behavior recorded. Puberty was defined as the first observed standing reflex for the back-pressure test. Estrous detection was discontinued at 226 days of age.

Only 60% (234) of gilts attained puberty by 226 days of age. Average age at puberty was 192 days. Figure 1 shows the distribution of gilts attaining puberty (blue bars) and the mean daily heat index (red line).

The distribution of gilts attaining puberty started to plateau around July 1, which coincided with a mean daily heat index of 85 to 90°F. Average length of estrus was 1.69 and 1.83 days at puberty (first estrus) and second estrus, respectively. Of the gilts that attained puberty, 71% exhibited a normal return to second estrus, while 16% had irregular returns to estrus and 13% did not return to estrus at all.

These results suggest that gilts from this genetic line experienced delayed puberty and irregular returns to service during warm temperatures. To reduce the impact of seasonality on reproduction, genetic suppliers should consider increasing genetic selection for reaching puberty and returning to estrus. Modifications to gilt housing could reduce the impact of warm temperatures.

Reduced seasonality in reproduction would reduce variation in conception rates, improve pig flow and perhaps reduce seasonality in market hog prices.

Researchers: Mark Knauer and Joe Cassady, North Carolina State University, Raleigh, NC. Contact Knauer at (919) 515-8797 or e-mail mtknauer@ncsu.edu.