Dietary Urea for Dairy Cattle. II. Effect on Functional Traits

Abstract

Rations containing urea were blended and fed free-choice during 2 trials to compare effects on performance. In groups 1, 2 and 3, respectively, urea N replaced about 0, 22 and 45% of plant N in heifer rations from insemination to mid-pregnancy, 0, 8 and 16% thereafter until 2 wk before calving, and 0, 18 and 36% (0, .9 and 1.8% of ration dry matter) precalving and during lactation. Dry matter of the lactation ration was 44% corn silage, 10% alfalfa bromegrass silage and 46% concentrate (corn, soybean meal or urea, minerals and vitamins). Trial 1 started with heifers before 1st insemination, and trial 2 started just prior to 1st calving. Dietary urea had no effect on mastitis or other infections, digestive disorders, ovarian function, services per conception and birth and survival of calves not aborted. Compared to group 1, group 3 had increased abortions of 1st pregnancy (trial 1), increased retained placenta at 2nd calving (trial 2), increased calving intervals (trial 2) and decreased gestation periods (both trials). Average daily yields of milk, fat-corrected milk, fat and protein were decreased 8, 7, 6 and 13% in group 3 compared to group 1 for 4 consecutive lactations of trial 2. Performance by group 2 was similar to group 1. Replacement values of urea should apparently be decreased if feeding rates approach 2% of the dry matter in a blended high energy ration fed free-choice.