Factors associated with return to estrus in first service swine females.

Reproductive failure is a major factor contributing to the culling rate of swine females. As return to estrus is the most frequent reproductive failure, the study of factors associated with its occurrence can help to identify females with a predisposition to this failure. This study was run in a farm with a housing capacity of 1500 Agroceres PIC females in southern Brazil. Logistic regression models were run to determine the relative contribution of some factors to the probability of a sow returning to estrus. Return to estrus occurred in 27.8% (10/36) and 10.7% (50/469) of females remated after a previous reproductive failure and first service females, respectively. Females with a previous reproductive failure had 3.2-times higher odds (P=0.003) of returning to estrus than first service females. In first service females (n=469), higher odds (P<0.05) of returning to estrus were observed in parity order (PO) 0 (14.3%; 3.1-times), PO1 (20.6%; 4.9-times) and PO2 (12.7%; 2.7-times) compared to PO>2 (5.0%; reference class) females. Weaned females with ovarian cysts, detected during estrus, had 7.6-times higher (37.5%; 3/8) odds (P=0.012) of returning to estrus than females without cysts (8.3%; 28/338). In the model run with 330 weaned sows, the return to estrus rate was not affected (P>0.05) by the number of weaned piglets (7-9; 10; 11 and 12 piglets) but sows with a lactation length (LL) of 15-19 days had 3.5-times higher odds (P<0.05) of a return to estrus than sows with LL of 20-21 days. Return to estrus in weaned sows was also affected by the interaction between PO and body condition score (BCS) loss during lactation. Loss of more than 0.5 point in BCS resulted in higher odds (P<0.05) of returning to estrus in PO1 and PO2 but not in PO>2 sows. Accurate detection of estrus in gilts and adequate feed intake during lactation of first and second parity sows are suggested to reduce return to estrus rate in younger females. To avoid the detrimental effect of short lactations on reproductive performance options such as increasing the lactation length, split-weaning or skip-mating may be considered.