Effects of Level of Concentrate on Utilization of Mature Prairie Hay by Steers and Performance of Beef Cows Grazing Native Winter Pastures of Differing Forage Availabilities

Two winter grazing trials (lasting from early December to late January) were conducted on consecutive years to determine effects of level of concentrate supplement and amount of forage available on performance of cows grazing dormant winter range. One hundred twenty (494 kg) and 126 (480 kg) dry, pregnant Simmental X Angus cows in year 1 and year 2 respectively were used in a 2 x 3 factorial arrangement with two pastures of differing forage availabilities and 3 levels of concentrate supplements. Year 1 supplements were combinations of soybean meal (SBM) and corn to provide .32 kg of CP from .87 (LOW), 2.22 (MED) and 3.57 kg (HIGH) of supplement. Year 2 supplements were similar but provided .23 kg of ruminally degraded protein from .76 (LOW), 2.24 (MED) and 3.60 kg (HIGH) of concentrate supplement. Four ruminal fistulated steers (570 kg) were used in a 4 X 4 Latin square design to examine effects of supplements (year 2) on forage intake, total tract digestibility, ruminal nutrient disappearance and ruminal pH when fed mature prairie hay. Supplements were fed to steers at proportional amounts on a BW-75 basis to what cows received. In year 1, a difference in forage quantity and quality was observed between high and low available forage pastures. Cows gained less BW (P\u3c.01) and lost more body condition (P\u3c.01) as corn in the diet increased. Interactions between level of concentrate and forage available suggest that HIGH was more detrimental to BW change (P = .1 0) and condition score change (P =. 07) when forage availability is low. In year 2, a greater amount of forage was available and no difference in forage quality was apparent between high and low forage pastures. Cows gained more BW (P\u3c.01) and body condition (P\u3c.01) as corn in the diet increased. This opposite response to year 1 may be attributed to a greater amount of available forage in year 2 and more ruminally degraded protein in higher levels of supplements. Results from the hay utilization trial indicate increasing amount of corn in the diet decreased forage intake (P\u3c.01) and digestibility (P\u3c.01) Total diet digestible DMI increased (P\u3c.01) as level of concentrate increased. Linear decreases in ruminal pH (P\u3c.01) and NDF disappearance (P\u3c.01) were observed as levels of concentrate increased