Utilization of urea and molasses supplements by cattle grazing cereal stubbles

Abstract

Responses of cattle to urea and molasses supplements while grazing cereal crop stubbles were studied in two experiments.In experiment 1,2.5-year-old Friesian cows in early pregnancy grazed an oat stubble (6800 kg DM ha-1) for 12 weeks at a stocking rate of 3.8 cows ha-1. In addition, some were offered liquid supplements of molasses, or urea-molasses. Liveweight gain (mean 18 kg) was not significantly different between treatments during the first 6 weeks. Thereafter, unsupplemented cows lost weight at 1.6 kg day-1 compared with 0.5 kg day-1 for supplemented cows (P(0.05).In experiment 2, 1.5-year-old Angus X Friesian cows in early pregnancy, and rumen-fistulated Friesian steers, were grazed on wheat stubble for 15 weeks and were unsupplemented, or offered supplements of a urea-mineral block, a molasses lick, or a urea-molasses lick. The stubble initially contained c. 6000 kg DM ha-1 of crop residues, and at all times at least 100 kg DM ha-1 of mainly green stemmy weeds of digestibility (0.45. The diet selected contained >50% dead plant material except during the final week, and ranged in digestibility from 0.53 initially to 0.31 in the 12th week.Intakes of all supplements were highly variable among individuals with coefficients of variation usually >50%. Mean intakes of digestible organic matter (DOM) were generally increased by supplementation.During the first 11 weeks, mean liveweight gains ranged from 3.5 to 27 kg for the unsupplemented and urea-mineral block fed cattle. respectively, but thereafter all cattle lost weight rapidly (mean 1.33 kg day-1).Mean concentrations of rumen volatile fatty acids (VFA) fell during the experiment from 80.3 to 47.4, and from 90.9 to 56.4 mmol l-1 in the unsupplemented and supplemented steers, respectively. Rumen VFA levels were not related to molasses or urea intakes, and overall there was no relationship between liveweight change and levels of VFA in the rumen. Mean rumen ammonia levels, which were generally < 100 mg N l-1 in unsupplemented and molasses-fed steers, were increased by both urea supplements, particularly in the second half of the experiment.Although various parameters indicated likely deficiencies of energy and N in the diet, these supplements were generally of little benefit to cattle grazing cereal stubbles.