Response of Holstein cows to replacing urea with with a slowly rumen released urea in a diet high in soluble crude protein

Ruminants have the ability to utilize non-protein N compounds as N sources for rumen microbial protein synthesis. The objective of this experiment was to determine if use of a slowly rumen released urea could increase productive performance and/or decrease urinary N components of dairy cows when used as a replacement for urea in diets high in soluble crude protein (CP). Nitroshure, a slowly rumen released encapsulated urea, which is 0.9 urea and 0.1 fat according to the manufacturer, had measured ruminal in sacco N release of 72, 89 and 99% of at 0.5, 4 and 12 h of ruminal incubation respectively. Four pens of multiparous lactating cows on a commercial dairy were fed one of two total mixed rations (TMR) formulated to supply 5% of ration CP from urea or encapsulated urea. There were two pens of early lactation cows and two pens of mid-lactation cows. The two experiments (by stage of lactation) consisted of two four week periods, and pen treatments were reversed in each lactation group after the first period. All cows were fed twice daily to appetite with daily intakes recorded by pen. The TMR were sampled at the beginning and middle of the last week of each experimental period and analyzed for nutrients. Cows were milked 3 times daily with milk yield and components measured on one day from each milking at the end of each experimental period. Representative urine samples were collected from 22 to 25 cows per pen that voluntarily urinated (at the morning ‘lock-up’ of cows for reproductive examination) on day 25 of each experimental period, with feces collected from the first 20 of those cows that were identified 24 h later at the same time of the day. There were no differences in TMR nutrient analyses between the urea and encapsulated urea supplemented diets. Dry matter (DM) intake did not differ between treatments, but early lactation cows gained from encapsulated urea addition as evidenced by an increase in milk fat (0.068 kg/d, P = 0.01) and protein (0.041 kg/d, P = 0.01) output versus the urea group. Milk energy output also increased by 4.2 MJ/d (P = 0.01) versus urea supplemented cows. No differences in milk production or components occurred in the mid-lactation group of cows, although numerical trends were similar to the differences in early lactation. Neither early nor mid-lactation cows had differences in urinary total, ammonia or urea N concentrations, or estimated urine output, due to treatment. Fecal digestibility of CP and neutral detergent fiber (aNDFom) were similarly unaffected by treatment in either group of cows. Feeding a slowly rumen released urea increased milk fat, protein and energy output in early lactation high producing dairy cows fed a diet high in soluble N, versus feeding an equivalent amount of urea on an N basis, but that it had little impact in mid-lactation cows. In contrast, substituting urea with a slowly rumen released urea had no impact on urinary N components, and calculated urinary output, of N compounds in either group of cows.