Digestion of fresh perennial ryegrass fertilized at two levels of nitrogen by lactating dairy cows

Two fertilizer levels (LN = 0 and HN = 80 kg N ha−1 before and after the first cut in spring 1991) were compared on a second cut (30 days old) of perennial ryegrass using a cross-over design with four fistulated cows producing 20 kg of milk at the beginning of the experiment. Grass was cut daily and given ad libitum in three meals per day. The absence of nitrogen fertilizer resulted in a reduction in the nitrogen content of grass from 24 to 17 g kg−1 dry matter. In addition, nitrogen intake was reduced from 367 to 263 g day−1, whereas organic matter intake (14 kg day−1) and milk output were not affected. When LN grass was fed, organic matter digestibility was slightly reduced, from 0.81 to 0.79 (P < 0.05) but the site of digestion was unaffected. Organic matter truly digested in the rumen was 0.94 of digestible organic matter intake. Fibre digestibility was reduced by 0.06 unit (P < 0.01) using the unfertilized grass. Non ammonia nitrogen supplied to the duodenum (366 and 349 g day−1 for HN and LN grass respectively) was slightly different (P < 0.05) between diets. Lower nitrogen fertilization affected neither microbial synthesis (21.2 g kg−1 digestible organic matter intake) nor microbial nitrogen flow and did not lead to any significant change in the amount of feed nitrogen escaping rumen degradation. Non ammonia nitrogen flow was equal to nitrogen intake in the fertilized grass but was substantially higher than nitrogen intake (+ 86 g day−1) in the unfertilized grass. This difference in net nitrogen recycling between the two diets matched the quantity of urea-nitrogen excreted in urine, which was reduced by 81 g day−1 using the unfertilized grass. The results suggest that although lowering the amount of nitrogen fertilizer alone has only a minor effect on dairy cow nutrition, it has a great effect on nitrogen metabolism, thus producing a much lower urinary nitrogen output.