Sows appear to eat more in lactation to compensate for lower feed intake levels during hot weather.

Seasonal infertility is prevalent in sows during summer months and is problematic for producers striving to maximize reproductive efficiency. While not all aspects of the condition are understood, it is universally recognized that the increase in wean-to-estrus interval (WEI) and anestrous are largely caused by higher ambient temperatures.

The objective of this study was to determine when, during the production cycle, sows are most sensitive to heat stress.

Fifty-eight Parity 1 Landrace and Landrace × Large White F1 sows were rotated through chambers in the Brody Environmental Center for 58 days. The test was initiated in late gestation, continued through lactation and culminated at rebreeding.

The ambient temperature treatments were either thermoneutral (TN) at 64-68°F. or heat stress (HS) at 75-86°F. for each of the three production phases. Therefore, treatments were applied in the gestation-lactation-breeding phases (20, 24 and 14 days, respectively) as follows:

15 sows: TN-TN-TN;

14 sows: TN-HS-TN;

14 sows: HS-TN-HS;

15 sows: HS-HS-HS.

Rectal temperature, body weight (BW), backfat (BF), loin eye area (LEA), feed intake (FI), piglet weights, WEI, ovarian follicular growth and subsequent breeding performance were measured. Data were analyzed for the effects of treatment and day.

Rectal temperature differed across phases and conditions (100.9°F. vs. 100.8°F.; 103.1 vs. 102.6°F.; and 102.2 vs. 102°F. for HS and TN during gestation, lactation and breeding, respectively.

Sows had similar feed intake (lb./day) when limit-fed during gestation (5 lb./day) and breeding (3.8 lb./day), but during lactation (ad libitum) TN sows had greater feed intake than heat-stressed sows (8.1 vs. 6.8 lb./day, respectively). Feed intake was greatest in HS-TN-HS treatment sows (8.7 lb./day). This suggests sows were able to eat more in lactation to compensate for the decrease associated with high temperatures previously in gestation.

There was no effect of treatment on body weight, backfat or loin eye area before farrowing (436.5, 0.98 in., 7.65 sq. in., respectively) or at weaning (372 lb., 0.78 in., 7.05 sq. in., respectively).

Total born (11.7), piglet birth weight (3.1 lb.) and total number weaned (10.3) were similar, but piglet weaning weight was greater for TN compared with HS sows (13.64 vs. 12.57 lb.). Wean-to-estrus interval (4.7 days), estrus duration (2.2 days), percentage of inseminated sows after weaning (86%), subsequent farrowing rate (90%) and subsequent total born (10.7 pigs/litter) were not different by treatment.

In summary, heat stress did not affect follicular growth, WEI, percentage inseminated, subsequent farrowing rate or total born. Decreased feed intake during lactation in heat-stressed sows may have led to lower piglet weaning weights, perhaps caused by a detrimental effect on milk yield. This project was supported by a grant from the USDA Cooperative State Research, Education and Extension Service.

Researchers: A.M. Williams, T.J. Safranski, D.E. Spiers, P.A. Eichen, E.A. Coate and M.C. Lucy, University of Missouri, Columbia, MO; contact Williams by phone: 765-404-6615 or e-mail: amw8v2@mizzou.edu.