The rate of disimilation of the dietary protein fraction of fresh forages, especially fresh legumes, is rapid and often exceeds microbial requirements for ammonia [Beever, 1993]. Consequently total dietary nitrogen [N] utilisation is impaired due to increased absorption of ammonia from the rumen and excretion of urea via the kidneys. This experiment, as part of a larger co-ordinated study to examine N utilisation in alfalfa swards for milk production, considered the evaluation of different isoenergetic supplements formulated to contain contrasting carbohydrate components with respect to starch and fibre level and type, and water soluble carbohydrate content.

Forty multiparous Holstein cows in immediately post peak lactation were randomnly allocated to one of four alfalfa paddocks, and provided fresh daily allowances [13kg Dry matter (DM)/cow/d] by means of electric fencing. Cows were milked twice daily [0600&1500h] when 3.5kg DM concentrate was offered at each milking. At all other times cows were at pasture and intakes were estimated by measuring pasture DM on offer to and refused by the cows.