Effect of Abomasal Infusion of Propionate on the GnRH-Induced Luteinizing Hormone Release in Prepuberal Heifers3

Abstract

Each of 12 prepuberal Brangus heifers (8 to 10 mo of age; 167 to 196 kg) was fitted with an abomasal cannula and assigned to one of two infusion treatments (six heifers/treatment): 1,150 ml of water/d (C) or 200 g of propionate (9% of digestible energy) plus 950 ml of water (P). The heifers were individually fed twice daily 4.50 kg of a diet consisting of 45% cottonseed hulls, 35% corn, 15% soybean meal, 3.5% molasses and 1.5% dicalcium-phosphate, salt and vitamins A, D and E. The infusion treatments were delivered continuously via a peristaltic pump throughout the trial. The heifers were given two 100-µg gonadotropin releasing hormone (GnRH) challenges, administered im 6 h apart, at three periods of the trial: Period 1-24 h after the start of the infusion treatment; Period 2-after at least 21 d from the start of the infusion treatment and Period 3-24 h after the infusion treatments were switched. Infusion treatments were switched immediately after sampling was completed at Period 2 and treatment was continued until sampling was completed at Period 3. Blood samples for luteinizing hormone (LH) were collected via jugular cannula prior to each initial GnRH injection and then at 10-min intervals for 12 h. At each period approximately 6 h after feeding, an additional blood sample was collected for determination of plasma glucose levels. There was no difference in the magnitude of the LH surge between C and P heifers at Period 1, aldiough P heifers had an LH surge of longer duration (P<.025) after the first GnRH injection than did C heifers. Propionate-treated heifers also had a lower concentration of plasma glucose (P<.10) than did C heifers at Period 1. At Period 2, the magnitude of the LH response to the first GnRH injection was greater in P than in C heifers (P<.005). Propionate-treated heifers also had a slightly higher peak LH concentration and a slightly greater area under the LH curve after GnRH injection than did C heifers (P>.10). Plasma glucose concentration at Period 2 was greater (P<.10) in P than in C heifers. At Period 3, P heifers switched to C maintained a greater LH response to both the first (P<.05) and the second (P<.025) GnRH injection than did C heifers switched to P. There was no difference in plasma glucose levels between C heifers switched to P and P heifers switched to C at this period. We conclude from these data that abomasal infusion of propionate enhances the ability of the prepuberal heifer to respond to a GnRH challenge. The design of the experiment did not allow us to distinguish between a direct propionate effect or a general energy level effect. Copyright © 1983. American Society of Animal Science . Copyright 1983 by American Society of Animal Science.