Level of Supplemental Fat in the Diet of Gestating Beef Cows: Effects on Cow and Calf Performance

Abstract This study evaluated the effects of feeding a canola seed-based (7.2% ether extract) pellet to gestating cows on cow performance, reproductive efficiency, milk yield and composition, and calf birth weight. Seventy-two pregnant, multiparous cows were allocated to 12 pens in a completely randomized block design. Body weight (BW; 605 ± 35 kg) was utilized as the blocking criteria. Diets were formulated to be isonitrogenous (CP: 8.20 ± 0.24% and 9.07 ± 0.05% second and third trimesters, respectively) and isocaloric (TDN: 58.29 ± 0.24% and 60.50 ± 0.04% second and third trimesters, respectively), and to meet NASEM (2016) nutrient requirements for pregnant beef cattle. Pens were randomly assigned to three replicated (n = 4) dietary treatments: 0 (CON), 150 (LOW), and 300 (HIGH) g of fat/d (DM basis). Dietary treatments were fed from 148 ± 8 d prepartum until at least 50% of all cows had calved, then switched to a common lactation diet. Body condition score (BCS), BW and subcutaneous fat thickness were measured at the start (SOT) and end of trial (EOT), and at 21 and 42 d postpartum. Orthogonal contrasts were used to assess linear and quadratic relationships between fat inclusion and dependent variables. At SOT, there were no differences (P ≥ 0.73) in conceptus corrected BW (597 ± 34 kg) or (P ≥ 0.13) BCS (2.85 ± 0.12, Canadian System: 1 to 5). After 84 d on trial (64 ± 8 d prepartum), conceptus corrected average daily gain (ADG) increased linearly (P = 0.04) as fat inclusion increased. After 151 d of supplementation (EOT), conceptus corrected ADG and subcutaneous fat thickness did not differ (P ≥ 0.71 and 0.63, respectively). BCS change from SOT to EOT increased, while EOT to 21 d postpartum decreased quadratically as fat inclusion increased (P = 0.02 and 0.01, respectively). Female calf birth weight decreased quadratically (P = 0.01) as fat inclusion increased, but there was no effect (P ≥ 0.75) on male calf birth weight. As fat inclusion increased, lactose concentration in milk decreased linearly (P = 0.04). As days in milk (DIM) increased, milk protein, total solids, and milk energy yield decreased (P ≤ 0.05). Milk urea-N tended to increase (P = 0.07), while 12 h milk yield (P = 0.05) tended to decrease as DIM increased. Milk fat and energy were not affected by fat inclusion, day, or the interaction between fat inclusion and day (P ≥ 0.16). There was a quadratic tendency for fat inclusion to affect pregnancy rate (P = 0.09), being maximized on LOW treatment. These results suggest that increasing fat inclusion during mid-to-late gestation tended to improve cow reproductive efficiency. Further investigation is required to determine the optimal level of fat inclusion from canola seed.