Effects of Post-Partum Lipid Supplementation and Source of Supplemental Lipid on Reproductive Performance of Lactating Beef Cows Grazing Cool-Season Grass Pastures

A three-year study was conducted to evaluate the effects of post-partum lipid supplementation and source of supplemental lipid on performance of lactating beef cows grazing cool-season grass (CSG) pastures. Each year, 36 second- and third-calving Angus cows with calves were stratified by BW, BCS (Scottish System: 1 to 5), and days post-partum (38±1.5 d), and randomly assigned to 9 paddocks (4 cows/pen) of long-established CSG pastures. Subsequently, each paddock was randomly assigned to one of three replicated (n=3) treatments: a non-supplemented control (CON) treatment, and two supplemented (SUP) treatments where cows were offered either a canola seed (CAN; 9.1±0.68% EE) or a flaxseed (FLX; 8.5±0.38% EE) based pellet. Pelleted supplements were offered daily and amounts were such that each paddock received 1.2 kg/d (300 g/cow/d) of lipids (EE) from supplement. Each year, the supplementation period was 42 d; after which, all cows were managed in a single group and exposed to a 63 d breeding season (1:18 bull:cow). Data were analyzed as a randomized complete block design with contrasts for the effect of lipid supplementation (CON vs. SUP) and source of lipids (CAN vs. FLX). At the start of trial, no difference (P≥0.42) was observed among treatments for BW (554±3.0 kg), BCS (95±0.02% of optimal cows), proportion of cows cycling (39±4.7%) and available CSG forage (1977±105.7 kg/ha). Over the 42 d of supplementation period, no difference (P≥0.56) was observed among treatments for nutrient composition of CSG pastures (12.5±0.24% CP and 41.5±0.45% ADF). However, CON had lower (P=0.04) residual forage (806±118.8 kg/ha) and tended (P=0.06) to have greater forage utilization (61±4.3%) compared to SUP, while no difference (P≥0.34) was observed between CAN and FLX (904± 88.8 vs. 971±88.8 kg/ha and 50±3.6 vs. 52±3.8%). At the end of trial, no difference (P≥0.69) was observed among treatments for BW (578±7.6 kg), ADG (0.6±0.12 kg/d), BCS (99±0.01% of optimal cows), and proportion of cows cycling (88±3.1%). Forty-five d after the end of breeding season, no difference (P=0.97) was observed among treatments for conception rate (97±1.6%). These results show that supplementing lipids to second and third calving beef cows prior to breeding has no effect on their reproductive performance. However, the lower forage utilization showed for supplemented cows suggests that this supplementation strategy might be suitable under limited forage and/or high stocking rates scenarios.