Effects of Sub-lethal Doses of Urea on Pregnant Cattle

Six pregnant cows received, by injection into the rumen, 0.20 to 0.30 g urea/kg body-weight at 104 to 148 days of pregnancy, 0.35 g/kg at 124 to 169 days of pregnancy, 0.35 g/kg at 208 to 226 days of pregnancy and 0.30 g/kg at 256 to 273 days of pregnancy. The change in blood ammonia concentration was followed for approximately seven hours after dosing. Five cows survived until calving and gave birth to healthy calves after gestation periods of 281 to 287 days. Thus, it appears that sudden access to a high intake of urea is unlikely to result in abortion or stillbirths. One cow receiving 0.35 g/kg at 208 days of pregnancy and a further two nonpregnant cows receiving 0.40 g and 0.45 g/kg under the same experimental conditions died from urea toxicity. In cows receiving non-fatal doses of 0.20, 0.25, 0.30 and 0.35 g urea/kg, the mean maximum increases of blood ammonia-nitrogen concentration were 3.1, 3.4, 6.1 and 6.0 pg/ml respectively. The three animals receiving fatal doses of 0.35, 0.40 and 0.45 g/kg had maximum blood ammonia-nitrogen increases of 27.1, 22.0 and 19.5 pg/ml at the final samplings 60, 13 and 15 min before death respectively.