Effects of season, outdoor climate and photo period on age at first observed estrus in Landrace × Yorkshire crossbred gilts in Thailand

The present study investigated the effects of season, outdoor climate and photo period on age at first observed estrus in Landrace × Yorkshire crossbred gilts in four commercial swine herds in Thailand. The gilts (n = 10,434) entered the gilt pools at 173.9 days of age (91.9 kg) and exited at 225.5 day of age (131.3 kg). Estrus detection was carried out after the gilts have entered the gilt pool by back pressure test with a fence line boar contact. Of these gilts, 64.0% (6677/10,434) exhibited standing estrus at least once before being sent to the breeding house. On average, the gilts exhibited first estrus at 31.6 ± 0.3 days after entering the gilt pool; and their average age at first observed estrus was 199.0 ± 0.4 days. Gilts exhibiting first standing estrus in the cool season (211.1 ± 0.5 days) were older than gilts exhibiting first estrus in the hot (208.7 ± 0.6 days, P = 0.012) and the rainy (207.4 ± 0.5 days, P < 0.001) seasons. However, the gilts entering the gilt pool in the cool season (31.4 ± 0.5 days) had a shorter interval from entry to first observed estrus than the gilts entering the gilt pool in hot (37.5 ± 0.5 days, P < 0.001) and rainy seasons (34.3 ± 0.5 days, P < 0.001). Gilts exposed to a high temperature, a high temperature humidity index (THI) or a short photo period during the 30 days before entering the gilt pool were older at first observed estrus than gilts exposed to a low temperature, low THI and/or long photo periods. An increase in photo period for 1 h resulted in a decrease in age at first observed estrus by 3.04 days (P < 0.001). Gilts with an old age at entering the gilt pool (i.e., delayed age at first boar exposure) were also older at first estrus than gilts with a young age at entering the gilt pool (r = 0.445, P = 0.005). Therefore, an intensive and careful estrus stimulation/detection should be performed in gilts entering the gilt pool in the hot season. Additionally, housing, lighting regimen and feeding management should be well prepared for young gilts that were expected to enter the gilt pool in the cool season. Moreover, age, body weight and growth rate of the gilts should be carefully determined before entering to the gilt pool.