Effect of Forage Species and Supplement Type on Rumen Kinetics and Serum Metabolites in Developing Beef Heifers Grazing Winter Forage

The objective was to determine the effect of stockpiled forage type and protein supplementation on VFA production, serum metabolites, and heifer BW. Over 2 yr, spring-born, crossbred heifers (n = 45) were stratified by BW at weaning and randomly assigned to 1 of 3 forage treatments: (1) endophyte-infected tall fescue (TF; Festuca arundinacea), (2) big bluestem (Andropogon gerardi Vitman) and indiangrass (Sorghastrum nutans L.) combination (BI), or (3) switchgrass (SG; Panicum virgatum L.). Forage treatments were then randomly assigned to receive 1 of 2 supplement types: (1) 0.68 kg·heifer^-1·d^-1 of dried distillers grains with solubles (DDGS: 28% CP, 74% RUP, 88% TDN) or (2) 0.22 kg·heifer^-1·d^-1 of blood meal and fish meal (BF: 72.5% CP, 67.5% RUP, 69.5% TDN), resulting in a 3 × 2 factorial arrangement of treatments. Treatments were initiated every year in January and terminated in April at the onset of a 60 d breeding season. Body weights and serum samples were collected every ~28 d from initiation of grazing until breeding. Serum glucose concentrations were greater (P = 0.05) for heifers grazing SG. Circulating NEFA concentrations were greater (P = 0.04) in TF heifers. Serum urea N (SUN) was not influenced (P = 0.37) by forage type. Ruminal acetate concentration and acetate:propionate ratio were greater (P = 0.04) in heifers grazing SG pastures. Heifer BW change from Jan to Feb and overall BW change were greater (P < 0.01) for TF heifers. However, BW change from March to April was not different (P = 0.84) among forage types. Serum glucose and SUN concentrations exhibited (P < 0.05) supplement type × sampling time interactions. Ruminal acetate tended to be greater (P = 0.09) and butyrate concentrations were greater (P < 0.01) for BF-supplemented heifers. Acetate:propionate ratio was not influenced (P = 0.15) by supplement type. Supplement type did not influence (P > 0.13) BW or BW change from Jan to Feb and Jan to April; however, heifers fed DDGS gained (P = 0.03) more BW from March to April. Heifer BW change from Feb to March exhibited (P < 0.05) a forage type × supplement interaction with BF heifers gaining more BW on BI pastures than DDGS heifers. Overall, developing heifers on low-quality native forages initially delayed BW gain; however, heifers seemed to quickly adapt to the lower-quality forage, resulting in similar BW gains in March and April.