Effect of monensin on the performance and nitrogen utilization of lactating dairy cows consuming fresh forage

We conducted a lactation trial with a fresh forage diet in order to evaluat e 1) the effects of monensin on nitrogen metabolism, and 2) the Cornell Net Carbohydrate and Protein System (CNCPS). Thirty Holstein cows in midlactat ion (eight fitted with ruminal fistulas) were gradually introduced to a fre sh forage diet. A concentrate mix based on corn meal was fed before the a,m , and p.m. milking times 0730 and 1730 h, then the fresh forage was fed at 0830 and 1830 h. Fifteen cows each were allocated to a control (no monensin ) and a treatment group receiving 350 mg/cow per day of monensin in the p.m . concentrate feeding. A 7-d fecal and urine collection period and a 3-d ru men sampling period were conducted with the fistulated cows. After the lact ation study was concluded, the fistulated cows were fed forage regrowth and a 3-d rumen sampling period was repeated. Monensin increased milk producti on by 1.85 kg. Milk fat and protein concentrations decreased and milk fat a nd protein yields increased, but the effects were nonsignificant. Monensin did not significantly affect DMI, Ruminal ammonia and the acetate-to-propio nate ratio decreased with the addition of monensin in both fed forages. Mon ensin decreased fecal N output, and increased apparent N digestibility by 5 .4%. Because of the decrease in ruminal ammonia and increase in apparent N digestibility, we concluded monensin was sparing amino acids from wasteful rumen degradation with a fresh forage diet. The precision of the CNCPS in p redicting performance was high (r(2) = 0.76), and the bias was low (overpre diction of 3.6%). These results indicate that the CNCPS can be used for dai ry cows consuming fresh forage and gives realistic predictions of performan ce.