Urea Utilization by Lactating Cows

Abstract

Efficiency of urea nitrogen utilization was determined in 6-day total collection digestibility and nitrogen retention trials conducted at three stages of lactation with four cows per diet in two trials. The cows had been fed their respective diets for an average of 1 yr (Trial 1) or 2 yr (Trial 2) prior to the study. Diets contained three percents urea of dietary nitrogen: 0 (control), 28 to 30 (low urea), and 40 to 45 (high urea). Corn silage was the only roughage offered. In Trial 1, the average daily absorbed nitrogen was greater for cows fed the high urea diet (203g) than for cows offered the low urea (172g) or the control (145g) diets; this was attributed to greater nitrogen intake. Average daily urinary nitrogen excretion of cows increased as quantity of urea in the diet increased: control, 76g; low urea, 113g; and high urea, 152g. Higher urinary excretion of nitrogen by cows fed urea diets was associated with lower productive nitrogen in midlactation (38g) and dry period (19g) but not in early lactation (107g). Apparent and true digestibility of protein was greater for the high urea diet (66.3%, 84.85%) than for the control (58.9%, 81.6%) or low urea (61.4%, 82.1%) diets. In Trial 2, absorbed nitrogen and productive nitrogen were not significantly affected by source of dietary nitrogen, but average daily urinary nitrogen excretion was greater for cows fed the high urea diet (121g) than for cows fed the control diet (97g). Differences among diets were not significant for apparent or true digestibility of protein. Dry matter, organic matter, and nitrogen-free extract digestibility of the control diet was greater than for other diets. Urea was effective as a partial substitute for dietary nitrogen protein in lactating cows fed urea daily for 2 to 3 yr. However, urea nitrogen was utilized less efficiently than nitrogen from soybean meal.