Effects of season, outdoor climate and photo period on age at first observed estrus in Landrace × Yorkshire crossbred gilts in Thailand Padet Tummaruk ⁎ Department of Obstetrics, Gynaecology and Reproduction, Faculty of Veterinary Science, Chulalongkorn University, Bangkok, 10330, Thailand article info abstract Article history: Received 25 May 2011 Received in revised form 15 November 2011 Accepted 17 November 2011 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 en- tered 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 b 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 b 0.001) and rainy seasons (34.3 ± 0.5 days, P b 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 b 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). There- fore, an intensive and careful estrus stimulation/detection should be performed in gilts enter- ing 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. © 2011 Elsevier B.V. All rights reserved. Keywords: Pig Reproduction Season Puberty Estrus Tropical climates 1. Introduction The reproductive efficiency of the gilts and the gilt man- agement are important factors driving reproductive perfor- mance and the longevity of sows in the swine herds. Age at first estrus, mating or conception in gilts is associated with their subsequent reproductive performance, longevity and the reasons for culling (Koketsu et al., 1999; Le Cozler et al., 1998; Patterson et al., 2010; Tummaruk et al., 2007). The per- centage of gilts culled due to reproductive disturbance in- creased from 18.0% to 24.5% when the age at first conception increased from 200 to 300 days (Schukken et al., 1994). Delayed age at first mating in gilts not only increases non productive days (NPD) from entry to conception, but also influences their reproductive performance as sows (Lucia et al., 2000; Tummaruk et al., 2001, 2007). Early pu- berty gilts have a shorter NPD from entry to conception than late puberty gilts (Patterson et al., 2010). Gilts with a delayed age at first mating (>260 days) have a shorter Livestock Science 144 (2012) 163–172 ⁎ Tel.: +66 2 2189651; fax: +66 2 2520738. E-mail address: Padet.T@chula.ac.th. 1871-1413/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.livsci.2011.11.010 Contents lists available at SciVerse ScienceDirect Livestock Science journal homepage: www.elsevier.com/locate/livsci