Late gestation supplementation with or without calcium propionate and rumen-protected methionine using nulliparous beef cows grazing dormant range forages

Beef production in the northern Great Plains and similar semi-arid environments depends on raising young beef females that can adapt to intra and interannual changes in forages and nutritional quality that accompanies these changes. The objective of the study was to determine the effect of rumen protected methionine and calcium propionate supplements fed to nulliparous beef cows on body weight, calf performance, and milk production. Four isocaloric and isonitrogenous supplements were formulated and fed at a rate of 908 g•cow -1 •d -1 for 101 d (Yr 1) and 127 d (Yr 2). Supplements contained 183 g CP and approx. 560 g TDN: 1) control (Con); 2) rumen protected methionine ( Met ; inclusion of 20 g•hd -1 •d -1 Mepron85); 3) calcium propionate ( Prop ; inclusion of 50 g•hd -1 •d -1 NutroCal®); 4) or rumen protected methionine and calcium propionate ( Met+Prop ; inclusion of 20 g•hd -1 •d -1 Mepron85®, and 50 g•hd -1 •d -1 NutroCal®). Cows from each supplement treatment were randomly assigned to pastures targeting one of two stocking rates to achieve either 40% (moderate) or 60% (heavy) utilization for two consecutive years. Nulliparous cows’ body weight at the termination of the supplementation period were similar ( P = 0.99) for supplemental treatment, however cows in the moderate stocking rate pastures tended to be heavier ( P = 0.06). A supplement × year interaction was measured for calf birth weight ( P = 0.04) with no influence derived from pasture stocking rates on calf birth weight ( P = 0.36). Calves born from cows that received the Met supplement were heavier in Yr 2 whereas calves in the Met+Prop treatment were heaviest in Yr 1 with all other treatment × year combinations being similar. A supplement × year effect was observed for peak milk production and energy corrected milk ( ECM , P = 0.01 and P = 0.03, respectively). Peak milk yield was 3.8 kg•d -1 lower for Con when compared to the Prop cows and ECM was 3.9 kg•d -1 lower for Con when also compared to the Prop cows in Yr 2 of the study, whereas all other treatment combinations within and across the two years were similar. These data indicate that rumen protected methionine and calcium propionate can augment dietary efficiencies and magnitude of responses are dependent on environment. • Moderate stocking rate improves nulliparous cows body weight when grazing senescent forages. • Metabolic enhancer calcium propionate improved milk production in first calf heifers. • Nulliparous cows require supplemental nutrients when grazing senescent forages. • Further research is needed to understand the amino acids requirements of young cows in a rangeland setting. • Methionine is not the only limiting amino acid for young cows grazing senescent forages.