The effect of feeding protected lipids on plasma progesterone levels in post-pubescent Holstein heifers during heat stress

Abstract

Animal Production in Australia 1998 Vol. 22 THE EFFECT OF FEEDING PROTECTED LIPIDS ON PLASMA PROGESTERONE LEVELS IN POST-PUBESCENT HOLSTEIN HEIFERS DURING HEAT STRESS P. ROUGH, P.J. GOODWIN, M.J. JOSEY and C. HANSEN Australian T ropical Dairy Institute, Dept of Animal Production, University of Queensland, Gatton, Qld 4345 Less heat is generated with digestion and metabolism of fat relative to carbohydrates. Consequently, feeding fat during periods of heat stress lessens the heat load on the animal and will increase the energy density of the diet during periods when feed intake is depressed (Thatcher 1995). Exposure to heat stress at breeding results in a lower rate of conception in dairy cattle. Distinct effects of feeding fat on enhancing ovarian follicular function in early lactation have been described (Lucy et al. 1991). Additional reports indicate that the type of dietary fat infused into the abomasum of lactating dairy cows alters the level of oxytocin-induced PGF2a from the uterus (Thatcher et al. 1994). It is hypothesized that the feeding of a protected lipid supplement will result in an increased availability of fatty acids for hormone production by the endocrine system, but will not alter the rumenally available energy density of the diet. By tailoring the dietary fat composition, there is thus a potential to regulate reproductive function. Nutritional management of the bovine may, therefore, offer a means of altering physiological responses that enhance reproductive efficiency and minimise the effects of heat stress. Twenty-eight Holstein heifers with active oestrus cycles were paired by weight and assigned to either a control or treatment group. The treatment heifers were fed a ration containing a protected lipid (linoleic acid; Rumentek, Moree, NSW) at the rate of 50g/head/day derived from cotton seed extract. All heifers were mated on their second cycle after initiation of the trial. Blood samples were taken three times a week from the time of first mating and continued for 60 days. Plasma samples were assayed for progesterone levels using the Coat-a-Count kit method (DPC, Los Angeles, CA). The time between oestrus cycles, as evidenced by both visual assessment and progesterone assays, was used to detect embryonic mortality. The data were analysed using the GLM procedures of SAS (1988). The results of the trial show that no significant difference in plasma progesterone levels could be detected between treatments for the first 24 days after mating. From days 25 to 60, however, the lipid fed group was found to have a significantly higher plasma progesterone level than the control group (P<0.01; Table 1). The interval between oestrus cycles indicated that two of the animals from the control group suffered early embryonic loss while the lipid fed group showed no such events. Table 1. Comparison of mean progesterone levels (ng/ml) between heifers fed a pr otected lipid supplement and control animals Diet Control Protected lipid * Significantly different (P<0.01) Mean progesterone Day 1 to 24 5.054 5.205 Mean progesterone Day 25 to 60 9.738 10.620* In conclusion, the feeding of a protected linoleic acid supplement to breeding heifers significantly altered plasma progesterone levels during days 25 to 60 after mating. This may decrease the amount of early embryonic loss in heat stressed animals. LUCY, M.C., STAPLES, C.R., MICHAEL, F.M. and T H ATCHER, W. W. (1991). J. Dairy Sci. 74, 483-489. SAS (1988). SAS Users Guide, Statistics. (SAS Institute Inc.: Cary, NC.). THATCHER, W.W., STAPLES, C.R., DANET-DESNOYERS, G., OLDICK, B. and SCHNITT, E.P. (1994). J. Anim. Sci. 72 (suppl. 3), 16. THATCHER, W.W. (1995). International Conference on Livestock Production in Hot Climates. pp. 55. (Sultanate Qabos University Press: Muscat, Sultanate of Oman). 279