Date of this Version
Published in Proceedings, Western Section, American Society of Animal Science (2009) Vol. 60: 360-363
A 2-year supplementation study conducted at Miles City, MT from mid-October to mid-December in 2007 and 2008 evaluated responses of beef cows (n = 141 in 2007, n = 138 in 2008; avg BW = 546 ± 5.2 kg) grazing dormant native range (8.8% CP, 64% NDF, 71% IVDMD) to two different supplementation strategies. Each year, cows were stratified by age and weight at weaning and then assigned to one of two supplements: 1) self-fed loose mineral mix (MIN) or 2) self-fed mineral plus high-bypass protein sources (MIN+PRO; 50% mineral mix, 25% feather meal, 25% fish meal). Target intakes were 70 g/d for MIN and 140 g/d for MIN+PRO. Cows were weighed and hip height and girth measurements were taken at the beginning and end of the 60-d studies. Weight-to-height and weight-to-girth ratio changes were calculated. Data were analyzed with supplement, cow age (2, 3, and 4+), year, and their interactions in the model. In 2007, cows fed MIN consumed 28 g/d and MIN+PRO cows consumed 93 g/d, which was lower than the target amount for both supplements. In 2008, MIN cows again failed to consume the target amount (13 g/d), while MIN+PRO cows consumed just over target amount (160 g/d). Cows lost similar (P = 0.70) amounts of weight during the study regardless of supplement treatment (-22 and -25 ± 5 kg for MIN and MIN+PRO, respectively). Likewise, weight-toheight ratio change (-0.25 and -0.25 ± 0.04) and weight-togirth ratio change (-0.10 and -0.12 ± 0.02) were similar (P ≥ 0.60) for MIN and MIN+PRO cows, respectively. Year × cow age interactions (P ≤ 0.08) were observed for weight change and weight-to-height ratio change. Two- and 3-yrold cows lost less weight in 2008 than in 2007, while mature cows lost similar amounts of weight in both years. All cows exhibited less change in weight-to-height ratio in 2008 compared to 2007, with the difference between years most pronounced in younger cows. Protein supplementation at this level did not impact cow performance; however, forage quality was higher than expected, which may have contributed to the lack of response to supplementation with the mineral-protein mix.