ABSTRACT
Summary Studies were conducted to assess the long-term effects of nitrogen (N) fertilization of tall fescue on cow-calf performance and health problems, including grass tetany, fat necrosis, gastrointestinal parasitism, and nitrate poisoning. Kentucky 31 tall fescue pastures fertilized at three levels of N were grazed with Angus cows at 0.4 ha/cow on a year-round basis. Fertilization levels included high N (667 and 0 kg N/ha/yr from broiler litter in 1968-1974 and 1975-1977, respectively); moderate N (134 and 224 kg N/ha/yr from ammonium nitrate [NH4NO3] in 1969 and 1970-1977, respectively); and low N (84 and 74 kg N/ha/yr from NH4NO3 in 1972 and 1973-1977, respectively). Within years, cattle and pastures were managed similarly. The overall mean available forage was 689, 1,287, and 2,167 kg/ha from 1973 to 1976 on the low-, moderate-, and high-N pastures, respectively. Mineral analyses of forage indicated a potential tetany hazard on all three pastures, yet from 1970 to 1977, 14, 9, and 0 cases of tetany occurred on the high-, moderate-, and low-N pastures, respectively. Mean cow weights differed among N fertilization levels and averaged 362, 410, and 443 kg for the low-, moderate-, and highN pastures, respectively. Respective actual weaning weights were 162, 182, and 188 kg. There was a tendency for lower (F = 0.11) conception rates (75%) on the moderate-N pasture than on the low- (90%) and high-N (86%) pastures. Supplementing cows with magnesium (Mg) during the grass tetany season increased adjusted 205-day calf weights across all pasture treatments and increased conception rate in cows grazing the moderate-N pasture from 46% to 79%. At the stocking rate of 0.4 ha/cow, fertilization with 224 kg N/ha/yr or greater increased grass tetany and fat necrosis problems. Nematode parasitism was associated with the quantity of forage available for consumption and consequently decreased as the level of N fertilization increased.
