Effects of a polymer-coated urea product on nitrogen metabolism in lactating Holstein dairy cattle.
Source
Department of Animal Science, University of Vermont, Burlington 05405, USA.
Abstract
The
purpose of this study was to evaluate the impact of polymer-coated urea
on nitrogen retention, rumen microbial growth, and milk production and
composition. Coated urea (CU) that is more slowly hydrolyzed to ammonia
than unprotected urea could potentially be used more efficiently by
rumen microorganisms. Eight cows were offered each of three diets in a
randomized crossover design. Each treatment period consisted of a 14-d
adjustment period and a 5-d collection period. Diets were formulated to
maintain milk production while reducing plasma urea nitrogen
concentrations and urinary nitrogen excretion. Diets consisted of corn
silage, mixed grass/legume haylage, chopped alfalfa hay, corn meal,
protein, vitamin and mineral supplements, in a total mixed ration and
fed ad libitum. The diets contained 17.9%, 18.1%, and 16.4% CP and 0,
0.77%, and 0.77% CU (dry matter basis) and are denoted as CP18-CU,
CP18+CU, and CP16+CU, respectively. Individual feed intakes were
measured, and total fecal, and urine collections were conducted. Cows
were milked twice daily at 0500 and 1700 h, and the milk sampled for
composition and milk urea N analysis. Dry matter intake averaged 23.5
+/- 0.2 kg/d and was not altered by diet. Also, milk fat and true
protein were not altered by diet and averaged 3.72 and 3.07%,
respectively. Milk yield was highest for diets CP18-CU and CP18+CU.
Significant differences were observed in N intake and excretion in
urine, feces, and milk between dietary treatments. Cows fed CP16+CU
consumed 11% less N than in CP18-CU. Cows fed CP18+CU showed the highest
excretion of N in urine, and together with CP16+CU, the lowest N
excretion in feces. Nitrogen excretion in milk was lower for cows fed
CP16+CU. Calculated N balance was not significantly different between
diets nor was it significantly different from zero. Efficiency of N
capture in milk protein as a function of N intake was higher for animals
on CP16+CU. Urinary excretion of purine derivatives was not different
between diets, and estimated microbial CP was also similar. Coated urea
was not effective at reducing nitrogen excretion by dairy cattle.
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