LEVEL AND SOURCE OF FAT IN THE DIET OF BEEF COWS

Two studies were conducted to evaluate the effects of fat and the type of fatty acid (MUFA vs. PUFA) inclusion in the diet of beef cows during the pre- and postpartum period on the performance of the dam and the progeny. In study 1, replicated over three years, 36 second- and third-calving lactating Angus cows were stratified by BW (554±15.5 kg) and days postpartum (38±1.5 d), and randomly assigned to 9 paddocks (4 cows/paddock) where cows grazed cool-season grass (CSG) pastures (12.5±2.5% CP and 56.5±2.9% TDN). Each paddock was randomly assigned to one of three replicated treatments: a non-supplemented control (CON), and two supplemented (SUP) treatments where cows were offered either a canola seed (CAN) or a flaxseed (FLX) based pellet targeting 300 g/cow/d of supplemental fat (EE) over 42 d. Data were analyzed as a RCBD with contrasts for the effect of fat supplementation (CON vs. SUP) and source (CAN vs. FLX). Results indicate that CON had greater (P=0.01) forage utilization and tended (P=0.08) to have greater estimated forage DMI compared to SUP, while no difference (P≥0.76) was observed between CAN and FLX. At the end of the trial, all treatments resulted in positive ADG, maintained or increased BCS and SCFT, and reduced serum NEFA concentration with no difference (P≥0.20) among treatments. No differences (P≥0.12) were observed for pregnancy rate, calving distribution and calving to calving interval. In study 2, replicated over 2 years, 75 multiparous (≥3 calving) pregnant Angus cows were stratified by BW (663±21.5 kg) and BCS (2.6±0.12), and randomly assigned to 15 outdoor pens. Subsequently, each pen was randomly assigned to one of three (n=5) treatments: a low-fat diet (LF; 1.4±0.12% EE) and two high-fat diets (HF; 3.3±0.20% EE) which included a CAN or a FLX pelleted feeds similar to those used in study 1. Diets were formulated to meet the requirements of pregnant beef cows during the last two trimesters of gestation (183±4.8 d), and offered such that each pen on average received similar amounts of DE (31.2±2.8 Mcal/cow/d), CP (1.36±0.13 kg/cow/d), and DM (12.9±1.0 kg/cow/d). Data were analyzed as RCBD with contrasts for the effects of level (LF vs. HF) and source (CAN vs. FLX) of fat. After 160 d on trial, conceptus corrected-BW (CC-BW) of LF cows (708 kg) and the proportion of over conditioned cows (13.2%) were greater (P≤0.04) than those of HF, with no difference (P≥0.84) between CAN and FLX. Feeding FLX diet over gestation resulted in subcutaneous adipose tissue (SCAT) with greater (P≤0.01) concentration of CLnA (0.12 vs. 0.05%) and n-3 (0.58 vs. 0.37%) fatty acids, and a tendency (P=0.09) for CLA concentration (1.05 vs. 0.88%) to be greater when compared to CAN diet. By the end of gestation, serum NEFA concentration of LF cows (592 μEq/L) was lower (P<0.01) than that of HF cows, and FLX cows had greater (P<0.01) serum NEFA concentration than CAN cows (636 vs. 961 μEq/L). Cows receiving the LF diet over gestation gave birth to lighter (P≤0.01) calves compared to those receiving the HF diets (40.2 vs. 42.9 kg), with no difference (P=0.24) between calves born to CAN and FLX cows. No differences (P≥0.21) were found for BW or calving to weaning ADG of cows. The average BCS during the first 42 d of lactation was greater (P<0.01) for LF compared to HF (2.63 vs. 2.51) with no difference (P=0.35) for CAN vs. FLX cows. Subcutaneous fat thickness over the 12/13th ribs was greater (P≤0.01) for LF compared to that of HF cows at calving (5.7 vs. 4.3 mm) and at weaning (4.3 vs. 3.7 mm) with no difference (P≥0.11) between CAN and FLX cows. Over the first 42 d of lactation, no difference (P≥0.23) was observed for 12-h milk yield. However, milk protein concentration was greater (P=0.03) for CAN compared to FLX (3.11 vs. 3.01%) cows while no difference (P≥0.28) was observed for any other milk component. Milk fat from FLX cows had greater (P