Urea for Lactating Dairy Cattle. V. Concentrate Fiber and Urea in a Corn Silage-High Concentrate Ration

Abstract

Four mature, lactating Holstein cows were used in a 4×4 Latin-square design to evaluate high protein rations containing two concentrations each of urea and fiber. Corn silage was offered ad libitum and diets were fed at 100 to 110% of energy requirements. Ration crude protein averaged 114% of requirements, and a 40:60 forage-to-concentrate ratio was employed.Increasing concentrate fiber from 4.6 to 8.2% significantly reduced dry matter, energy, and fiber digestibilities as well as total digestible nutrients and metabolizable energy of the ration without affecting dry matter, energy, or concentrate intake. Energy losses in urine, methane, milk and heat production were not influenced by fiber concentration. There was a trend for higher intake to decrease nitrogen retention, but it did not affect the loss of nitrogen in feces, urine, or milk. Fiber did not influence proportions of ammonia or volatile fatty acids in rumen fluid just before or within three hours after feeding.Incorporation of 2.5% urea in the concentrate in place of plant protein increased urinary nitrogen excretion and reduced efficiency of nitrogen utilization for milk and body retention by about 20%. Urea increased the molar proportion of propionate and decreased the proportion of butyrate in rumen fluid two hours postprandial. The other traits studied were not affected appreciably by inclusion of urea in the concentrate. Energy and nitrogen partition data are compared with previous reports.