40. Strategic Protein Supplementation with Gestating Beef Cows Grazing Winter Foothill Rangelands Across Four Years.

Abstract

Protein supplementation is widely used to mitigate the negative effects of low-quality, dormant forage on body weight (BW) and body condition score (BCS) in gestating beef cows grazing during the winter. However, the responses of grazing cows to increasing supplemental protein supply and to different supplement sources under extensive rangeland conditions remain poorly defined. Cow responses to supplementation may also vary across winters, yet few studies have evaluated these responses across multiple years. Therefore, the objective of this study was to evaluate the effects of graded levels of supplemental protein on winter BW and BCS of gestating beef cows grazing dormant native foothill rangeland across multiple years. A herd of multiparous, Angus-based, May-calving cows was used in four 84-d winter grazing experiments conducted during mid to late gestation in southwest Montana. During the winters of 2020-2021 and 2021-2022, 180 cows were stratified by age and body condition and, within stratum, randomly assigned to one of three levels of a 30% CP canola-based pelleted supplement receiving 1) no supplement, 2) 0.17 kg·d⁻¹, or 3) 0.35 kg·d⁻¹. During the winters of 2022-2023 and 2023-2024, 240 cows were stratified by age and body condition and, within stratum, randomly assigned to one of four levels of a 18% alfalfa-based supplement: 1) no supplement, 2) 0.14 kg·d⁻¹, 3) 0.27 kg·d⁻¹, or 4) 0.41 kg·d⁻¹. All four years, cows were gathered three times weekly and provided supplement on a replicated pen basis. Body weight and BCS were recorded at 28-d intervals to quantify winter changes in cow condition in response to supplemental CP supply. Data were analyzed using mixed-effects models to evaluate the effects of supplemental CP intake, day of gestation, and year on winter body weight and BCS. Crude protein supplementation affected cow BCS and BW, and these responses varied with day of gestation (P < 0.001). For example, cows receiving no supplemental CP lost 70.28 kg BW and 0.60 BCS units over the 84-d feeding period, whereas those receiving moderate supplemental CP (0.14 kg·d⁻¹), lost 1.31 kg BW and lost 0.34 BCS units. Cows receiving higher supplemental CP (0.41 kg·d⁻¹) gained 33.79 kg·d⁻¹ and lost 0.08 BCS units over the same period. Day of gestation also influenced BW and BCS, and the magnitude of gestational change differed among years (P < 0.01). Cow age increased BW (P < 0.001) but did not affect BCS (P = 0.43). These results indicate that supplemental CP supply interacts with physiological status and annual environmental conditions to influence winter performance of gestating beef cows grazing dormant rangeland. Strategic protein supplementation may therefore help mitigate seasonal losses in BW and BCS under extensive winter grazing systems.