Reproductive performance and serum fatty acid profiles of underdeveloped beef heifers supplemented with saturated or unsaturated rumen bypass fat compared to an isocaloric control

Research involving fat supplementation to heifers has shown positive reproductive effects, but the effect of lipid composition has had little investigation. Heifers (n = 118) were blocked by age, breed, and BW and pen-fed alfalfa hay. Heifers were individually fed 1 of 3 isocaloric supplements: control (CON) or either half the CON diet with 0.2 kg of a rumenprotected unsaturated fatty acid source (USFA) or a rumen-protected saturated fatty acid source (SFA) fed 5 d/wk. Heifers received treatments for 3 wk before starting the 7-d controlled intravaginal drug release estrus synchronization protocol. Heifers remained on treatment for 28 d after AI. Pregnancy was determined at d 30 post-AI via transrectal ultrasonography and by rectal palpation at 150 d post-AI. Serum progesterone and leptin were measured by RIA and serum triglycerides; cholesterol was measured by colorimetric procedures. Heifers BW gain during supplementation was similar (P = 0.35) between treatment groups (32 ± 2 kg). Percentage of heifers cycling tended to be less (P = 0.08) for USFA heifers compared with CON and SFA treatments (36 vs. 55 and 56%, respectively). Pregnancy rates by AI of heifers detected in estrus were similar (P = 0.35) between treatments. Serum total and specific fatty acids, cholesterol, and triglycerides were greater (P < 0.05) at d 21 and 56 in fat-supplemented heifers compared with CON heifers. At d 56 of treatment, USFA had greater (P < 0.05) plasma leptin compared with SFA, with CON heifers having further reduced serum leptin concentration. Feeding rumen-protected fats to heifers increased circulating lipid and leptin, but did not influence reproduction rates.