Animal Reproduction Science 99 (2007) 167–181 Age, body weight and backfat thickness at first observed oestrus in crossbred Landrace × Yorkshire gilts, seasonal variations and their influence on subsequence reproductive performance P. Tummaruk ∗ , W. Tantasuparuk, M. Techakumphu, A. Kunavongkrit Department of Obstestrics Gynaecology and Reproduction, Faculty of Veterinary Science, Chulalongkorn University, Bangkok 10330, Thailand Received 17 June 2005; received in revised form 19 April 2006; accepted 11 May 2006 Available online 9 June 2006 Abstract The objective of the present study was to investigate puberty attainment in crossbred Landrace × Yorkshire (LY) gilts reared under tropical conditions and their subsequent reproductive performance. This study was carried out in a 2400-sow herd over a 1-year period. A total of 696 crossbred LY replacement gilts were included. Faecal samples from 214 gilts were collected to determine the faecal progesterone profiles around the time of first oestrus. Solid-phase 125 I-radioimmunoassay was used to determine the progesterone concen- trations in the faecal extract. The gilts entered the herd at an average age of 177.5 ± 12.6 days, 95.7 ± 10.2 kg body weight (BW) and a backfat thickness (BF) of 12.0 ± 2.9 mm. On average, the gilts expressed first stand- ing oestrus at 195 days of age, 106 kg of BW and a BF of 13.0 mm. The interval from entry to the gilt pool to the first observed oestrus (EOI) was 24.4 ± 18.0 days (range 0–88 days). The hormonal profile indicated that the gilts that actually ovulated during the first observed oestrus was 34% (group A), the gilts that had ovulated before the first observed oestrus was 21% (group B) and the gilts that did not ovulate during the first observed oestrus was 45% (group C). During summer the proportion of group A gilts was significantly lower than during the winter and the rainy seasons (P < 0.05). The BW of gilts at entry significantly correlated with the BF at entry (r = 0.31, P < 0.001), the age at entry (r = 0.47, P < 0.001), the BW at first oestrus (r = 0.65, P < 0.001) and the BF at first oestrus (r = 0.33, P < 0.001). An increase of BW at entry of 1 kg resulted in a decrease of EOI of 0.28 days. The age, BW and BF of gilts at the first observed oestrus significantly influenced the total number of piglets born per litter (TB) and the number of piglets born alive per litter (BA) in the first three parities. Gilts expressing their first oestrus between 181 and 200 days had a signif- icantly larger TB than gilts that expressed first oestrus between 150 and 180 days (P = 0.03) and between ∗ Corresponding author. Tel.: +66 22189644; fax: +66 22520738. E-mail address: Padet.T@chula.ac.th (P. Tummaruk). 0378-4320/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.anireprosci.2006.05.004