Postpartum Nutrient Intake and Body Condition: Effect on Pituitary Function and Onset of Estrus in Beef Cattle

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Abstract
From calving through first estrus, 30 Brangus females were assigned equally to one of three diets to study the effect of postpartum nutrient intake and body condition on the ability of the pituitary to release luteinizing hormone (LH) and on the postpartum interval to estrus (PPI). The postpartum diets were calculated to achieve a 1) low [90% of the National Research Council (NRC) recommendations], 2) maintenance (100% of the NRC recommendations), or 3) high (110% of the NRC recommendations) level of nutrient intake. The females were group-fed within a treatment and calves were allowed to suckle ad libitum. Cow weight, body condition score and calf weight were recorded 24 h aftercalving, d 20 postpartum and at first behavioral estrus. On d 21 postpartum, blood samples were collected via jugular cannulae at 15-min intervals for 4 h, followed by a 100-/µg im injection of gonadotropin-releasing hormone (GnRH) and continued sampling at 15-min intervals for an additional 6 h to determine serum LH. Although there was a significant decrease in PPI with increasing levels of nutrient intake (low = 57.5 ± 8.8 d; maintenance = 40.3 ± 6.6 d; high = 34.7 ± 5.1 d), there were no differences in any of the observed LH characteristics (or variables) due to treatment. There were, however, marked differences in both the PPI and LH characteristics when data were analyzed on the basis of ability to maintain, body condition from calving through 20 d postpartum, regardless of calculated dietary treatments. Cows that maintained body condition (MBC) had a shorter PPI [MBC, 31.7 ± 2.8 vs lost (LBC) 60.0 ± 7.5 d; P<.01], higher basal levels of endogenous LH (MBC, .83 ± .09 vs LBC, .61 ± .04 ng/ml; P<.025), higher GnRH-induced peak LH concentration (MBC, 58.99 ± 11.15 vs LBC, 38.86 ± 8.37 ng/ml. P<.10), higher LH levels throughout the GnRH-induced LH surge (P<.001), and greater release curve areas for the endogenous (MBC, 124.6 ± 13.3 vs LBC, 91.7 ± 5.6 units; P<.025), GnRH-induced (MBC, 4370.8 ± 699.5 vs LBC, 3039.7 ± 683.3 units; P<.10) and total (MBC, 4510.7 ± 706.7 vs LBC, 3141.9 ± 684.7 units; P<.10) LH release. Results from this study suggest that females maintaining body condition after parturition, regardless of calculated nutrient requirements, have an enhanced pituitary function and reproductive potential. Copyright © 1984. American Society of Animal Science . Copyright 1984 by American Society of Animal Science.