EFFECT OF LACTATION FEED INTAKE AND GONADOTROPHIN-RELEASING HORMONE ON THE REPRODUCTIVE PERFORMANCE OF SOWS

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Abstract
Twenty-four first parity and 48 multiparity crossbred sows (Yorkshire × Landrace) were used in a 2 × 2 factorial design involving high (H) or low (L) lactation feed intakes and the intramuscular injection, or not, of 50 μg of gonadotrophin-releasing hormone (GnRH) at the onset of the first postweaning estrus. All sows were weighed at farrowing and weaning. The H feed intake entailed feeding sows at levels of 10, 13, and 14% of each sow's immediate postfarrowing metabolic weight during weeks 1, 2, and 3–4 of lactation, respectively. Low-fed sows received 50% of the H feed level. The diet contained 12.5 MJ DE kg−1 and 16% crude protein. Between weaning and mating, and following mating, all sows were fed 2.25 kg daily of the lactation diet. After weaning, sows were exposed to a boar twice daily to facilitate estrus detection. GnRH was administered at the onset of standing heat. All sows were slaughtered 25 d after mating at which time the reproductive tracts were removed and examined to determine the number of ovulations and the number of viable embryos. Low-level feeding resulted in an extension of the remating interval (5.9 ± 0.3 vs. 4.5 ± 0.2 d; P < 0.05), an increased incidence of anestrus (16.7 vs. 3.6%; P > 0.1) and a reduction in pregnancy rates (69.7 vs. 81.6%; P > 0.1). Interactions were noted between lactation feeding level and GnRH injection (P < 0.05) and between parity and GnRH injection (P < 0.05) for number of viable embryos. The data obtained suggest that both low level of feeding during lactation and a young age are associated with decreased embryo numbers, but this situation is alleviated by the administration of GnRH. Key words: Sows, feed intake, estrus, GnRH