Effect of crude protein source on in vitro ruminal ammonia accumulation and nitrogen partition in dairy cows

Abstract

Two experiments evaluated the effect of crude protein (CP) sources on N-NH3 accumulation in rumen fluid in vitro and on lactation performance, dry matter (DM) intake (DMI), digestibility, eating and rumination behaviors, urea-N in milk and plasma, and N partition of lactating cows fed diets with excessive N supply. We evaluated the effect of products composition and particle size on nitrogen (N) release. Particle size fractions of ESU200 with variation in CP to starch ratio caused similar N-NH3 accumulation in vitro. The N-NH3 accumulation in vitro was highest for urea, lowest for

soybean meal (SBM) and intermediate for wax-coated urea (WCU) and extruded starch- urea with 138% and 210% CP (ESU200). Sixteen Holstein cows (159 ± 117 days in milk)

were individually fed a sequence of 4 treatments in 4×4 Latin squares, with 21-d periods, and 14 d of adaptation. Treatments were: Control (CTL, 16.4% CP in DM) and 3 diets with 18.1% CP in DM with added N from SBM, urea, or ESU200. Plasma urea-N (PUN) was measured at 0, 1.5, 3, 6, 9, 12, 15, and 21 h post-feeding. Partition-N into milk, urine, and feces and diet digestibility were measured by total collection of urine and feces. Rumen microbial yield was estimated by allantoin in urine. Treatment means were compared by tukey test (P ≤ 0.05). Treatments had no effect on DMI, milk yield, rumen microbial yield, and diet digestibility (P > 0.28). Diet SBM increased milk protein yield relative compared to CTL and ESU200 reduced milk fat yield relative to SBM and Urea. High protein diets increased the proportion of daily intake at night and had no effect on eating and rumination behaviors. The milk urea-N (MUN) and PUN were lowest on CTL (15.6 and 17.1 mg/dL), highest on ESU200 (21.3 and 22.5 mg/dL), and intermediate on SBM (19.3 and 21.6 mg/dL) and urea (19.4 and 21.2 mg/dL). The pattern of PUN variation over time differed by treatment (P < 0.01 for the interaction between treatment and sampling time). Cows fed high CP had lower N efficiency for lactation (22.9 vs.

25.2% of N intake) and excreted more urea-N in urine (162 vs. 121 g/d) than CTL. Urine- N was higher on urea (317 g/d and 43.5% of N intake) than on CTL (246 g/d and 37.3%

of N intake) and SBM (274 g/d and 36.9% of N intake), and intermediate and with no difference from the other 3 diets on ESU200 (300 g/d and 41.0% of N intake), reflecting the ranking of feeds based on N-NH3 accumulation in vitro. Excessive dietary CP supply reduced the efficiency of N utilization for lactation and increased urine-N, independently of CP source.