Performance of females heterozygous for deficiency of uridine monophosphate synthase.

Milk orotate concentration was tested for 1451 registered and grade Holsteins from 23 herds. Cows with milk orotate concentration exceeding 200 micrograms/ml were blood-tested for uridine monophosphate synthase activity. Thirty-four cows were diagnosed as deficient in this enzyme, which is necessary for the metabolic conversion of orotate to uridine 5'-monophosphate. To control for genetics, a subset of 33 deficient cows and their 145 normal paternal half-sisters were also analyzed. Milk yield at 305 d, adjusted for age and milking frequency, was significantly affected by the stage of lactation at which the test was done, by the parity of the cow, and by the interaction of the parity of the cow with its diagnosis of uridine monophosphate synthase in both analyses. Twenty-five older, deficient cows yielded the most milk (8118); 8 first-calf, deficient cows the least milk (6810); and 1386 normal cows intermediate amounts. Deficiency effect and interaction of parity with deficiency were not significant for fat. Interaction of parity with deficiency was significant for calving interval. Older, deficient cows had the longest calving intervals; first calf, deficient cows had the shortest calving intervals; and normal cows had intermediate calving intervals. However, deficiency was not associated with milk yield in analysis that considered calving interval as a covariate for milk.