Influence of energy and biological type on GnRH-induced LH release and plasma progesterone, estrone and estradiol concentrations in pre- and postpartum beef heifers

Typescript (photocopy).Forty-six nulliparous beef heifers (16 to 23 mo) of two biological types (SMALL = Red Poll-sired, LARGE = Charolais-sired) were individually fed from d 90 of gestation through parturition. The pregnant heifers were allotted to one of two nutritional treatments to achieve a reduction in initial weight (LOSS, fed 1% of weight/d) or a maintenance of weight (MAINT, fed 1.5% of weight/d) to parturition. Condition score (1-thin, 9-fat), hip height and weight were recorded every 28 d. Heifers were injected (im) with gonadotropin-releasing hormone (GnRH, 100 ug) during gestation (PD1, d 90; PD2, d 180; PD3, d 270) and postpartum (PD4, d 3 to 10; PD5, d 28 to 35). Blood samples were collected preceding GnRH injection and at 20-min intervals to quantify luteinizing hormone (LH) for 4 h after GnRH injection. Progesterone (P), estrone (E) and estradiol concentrations (E2) in plasma were quantified in samples obtained at 1-h, 2-h and 2-h intervals, respectively. Weight change from d 90 to parturition differed (P<.01) between LOSS (-17.7%) and MAINT (-2.4%) heifers. Least squares means (LSM) for P concentrations differed (P<.05) at PD1 and PD3 between SMALL and LARGE heifers and at PD5 between LOSS and MAINT heifers (P<.05). The concentration of E1 and E2 differed (P<.05) between SMALL and LARGE heifers at PD2. There was a positive correlation (r = .76 to .97, P<.01) between E1 and E2 for all periods (PD1 to PD5). Least squares means for peak LH (ng/ml) were greater (P<.05) in LOSS than in MAINT heifers in the two periods preceding parturition (PD2 and PD3). Inverse relationships for metabolizable (ME) and retained energy (RE) with LH response were detected during PD2 and PD3. Change in condition score (fat reserve) from 90 to 270 d gestation and age of dam accounted for more of the variation in LH response to GnRH at PD3 than other variables. Biological type affected P during gestation (P<.05 at PD1 and PD3) but not postpartum. Nutrient restriction during gestation increased pituitary LH response to exogenous GnRH prior to parturition in heifers of both biological types. This response was not due to changes in plasma P, E1 or E2. Energy balance components during gestation are significant factors in mediating the GnRH-induced LH response in beef heifers prepartum