An Investigation into the Effect of Body Condition and Other Gilt Characteristics on Estrus Responses Post Altrenogest Treatment and on Reproductive Performance

This study was conducted to investigate the effect of body condition (BC) and other characteristics on estrus responses post altrenogest treatment and on reproductive performance in 161 gilts, bred artificially, treated with 20 mg altrenogest orally for 18 consecutive days. Parameters recorded at the beginning of altrenogest treatment included age and body weight. Backfat (BFT) and muscle thickness (MT), both defining BC, puberty status and uterine size (UtS, based on cross-sectional areas of the uterine horns) were recorded by real-time-ultrasound (RTU), and BFT, along with MT together with ovary status, were recorded again at the end of altrenogest treatment. Estrus rate (ER), interval of last altrenogest treatment onset of estrus (ATOE), conception (CR) and farrowing (FR) rates, litter size characteristics (total, life, and still born piglets; TB, LB, SB) and individual piglet birth weights (IPWs) were recorded. Average age and weight were 252.6 ± 7.5 days and 153.0 ± 12.4 kg, respectively. BFT and MT were 0.9 ± 0.2 and 6.0 ± 0.4 cm, respectively, at the beginning of altrenogest treatment, and similar at the end. 147 gilts were determined as pubertal (PUB) and 14 prepubertal (PRE). All gilts only had follicles of 3.2 ± 0.4 mm on average at the end of altrenogest treatment. ER, ATOE, CR, FR, TB, LB, SB and IPW were 99.4%, 5.9 ± 0.5 days, 100%, 95.6%, 18.1 ± 2.6, 17.4 ± 2.5, 0.7 ± 1.0, 1.3 ± 0.2 kg, respectively. PUB and PRE did not differ, or only marginally differed, in all recorded parameters. BC or pubertal status did not have a great impact on the efficacy of altrenogest treatment or reproductive performance. However, UtS was significantly smaller in PRE than in PUB (0.51 ± 0.15 cm² and 1.63 ± 0.48 cm², respectively). There was a very slight significant relationship between UtS and LB (r = 0.19, <i>p</i> = 0.03) and a tendency towards TB (r = 0.15, <i>p</i> = 0.09). In conclusion, under the conditions of this study, BC, age and pubertal status did not have an effect on the efficacy of altrenogest or the reproductive performance of hyper-prolific gilts of Danish genetic origin, but uterine size may have had a positive effect.