Age, body weight and backfat thickness at first observed oestrus in crossbred Landrace × Yorkshire gilts, seasonal variations and their influence on subsequence reproductive performance

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 125I-radioimmunoassay was used to determine the progesterone concentrations 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 standing 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 significantly larger TB than gilts that expressed first oestrus between 150 and 180 days (P = 0.03) and between 201 and 220 days (P = 0.003). Gilts that showed first oestrus between 110.1 and 120.0 kg had a larger TB and BA than gilts that showed first oestrus between 80.0 and 100.0 kg (P < 0.05). Gilts that showed first oestrus with a BF between 13.1 and 15.0 mm had a larger TB and BA than gilts that showed first oestrus with a BF between 11.1 and 13.0 mm (P < 0.05). Group A gilts had a significantly larger TB than group B (10.5 piglets/L versus 9.4 piglets/L, P = 0.02), while farrowing rate (FR) did not differ significantly among groups A, B and C (78.1, 76.9 and 77.6%, respectively). Gilts that farrowed in the summer had a larger TB and BA than gilts that farrowed in the winter (TB, P = 0.03; BA, P = 0.09) and the rainy season (TB, P = 0.006; BA, P = 0.003). In conclusion, LY gilts reared under tropical conditions expressed first standing oestrus at 195 days of age, 106 kg BW and a BF of 13.0 mm. Under field conditions, 21% of the gilts with an observed oestrus had ovulated. The proportion of gilts that showed first oestrus and ovulated normally was lowest during the summer. The age, BW and BF at first observed oestrus influenced subsequent reproductive performance over the first three parities. The mean litter size (TB and BA) in the first three parities were highest in gilts that had a first observed oestrus between 181 and 200 days with 110.1–120.0 kg BW and 13.1–15.0 mm BF.