Responses to exogenous gonadotrophin treatment in contemporary weaned sows.

The principal goal of commercial breeding herds is to consistently meet weekly breeding targets. Weaned sows failing to return to estrus within 7 d after weaning contribute to missed breeding targets and increased non-productive sow days. Treatment with low doses of exogenous gonadotrophins (GT) has traditionally been used to advance and synchronize estrus in weaned primiparous sows. However, in well managed contemporary commercial sow farms, more than 90% of sows may return to estrus within 7 d after weaning, posing questions about the likely efficacy of exogenous GT treatment. Therefore, the primary objective of the present study was to determine the response to GT treatment at weaning in contemporary parity 1 commercial sows with lactation lengths typical of the North American swine industry. Primiparous crossbred sows (PIC C22, n = 275; and PIC C29, n — 131) from a 5,000 sow commercial farrow-to-wean facility (Wildcat, Carthage Veterinary Service, Carthage, IL) were blocked by estimated farrowing weight and genetic line and then randomly allocated to either receive a combination dose of 400 IU eCG and 200 IU hCG (PG600, Intervet, USA, De Soto, KS) I.M. in the neck on the morning of weaning (PG group; n = 189), or to be untreated controls (CON group; n = 218). From the day after weaning, all sows were provided twice daily fence-line contact with mature boars for stimulation and detection of estrus. Sows were bred according to established herd protocols with doses of 3.0 x 109sperm cells/insemination. Reproductive parameters analyzed were estrus synchronization rate (ESR),determined as the number of sows with observed standing heat within 7 d after weaning, weaning-to-estrus interval (WEI), proportion of sows bred over a 3-d period (BRD3), proportion of sows bred that farrowed (FR), total litter size (TB), and total live born piglets (BA) at farrowing. Based on WEI, sows were retrospectively grouped into 4 categories (WCAT); 1) Target Breed Week: sows bred within 7 d post-weaning, 2) "Tail-enders" sows with an extended 8-19 d WEI , 3) "Missed heats" sows detected in heat a 20 d post weaning, or 4) No detected heat by 30 d post weaning. Estimatedfarrowing (193.6 ± 1.5 vs 192.2 ± 1.6 kg)and weaning (189.4 ± 1.3 vs 188.0 ± 1.4 kg) weights were similar in CON and PG sows, respectively, and marginal weight loss in lactation was recorded across all sows (4.2 kg). Considering data from all sows assigned to treatment (Table 1), treatment did not affect the proportion of sows bred within 7 d after weaning (ESR),or within a 3-d breeding window (BRD3). However, the timing of this 3-d breeding window (d 4, 5 and 6 for CON vs d 3, 4 and 5 for PG sows) reflected a shorter (P < 0.001) WEI in PG compared to CON sows. PG treatment also eliminated the incidence of sows with an extended WEI (classified as "tail enders"), and reduced variance (P <0.05) in WEI in PG (0.07) compared to CON (0.14) sows when considered over the 19-d period after weaning. For those sows bred within 7 d after weaning, farrowing rate (82.8 vs 86.4%), total born (12.2 ± .3 vs 12.9 ± .3) and born alive (11.6 ± .3 vs 12.1 ± .3) were