Effect of protein content in the concentrate and level of nitrogen fertilization on the performance of dairy cows in pasture

The animal performance response to protein supplementation of grazing dairy cows was studied in relation to the level of nitrogen fertilization on swards. During two trials carried out in perennial ryegrass swards fertilized with a high (HN: 60 kg N/ha/cycle) or low (LN: 20 kg N/ha/cycle) level of nitrogen fertilization, comparisons were made between four isoenergetic doses of protected meal (PM) (trial 1.0, 25, 50 and 100% PM; trial 2: 0, 15, 30 and 60% PM). The trials were carried out in the spring (8 weeks) with 32 dairy cows each year, according to a 4 x 4 intrafertilization latin square design. The swards were grazed at the same age of regrowth for the two levels of fertilization. Reducing nitrogen fertilization induced a sharper decrease in sward productivity in the first trial, as well as the foreseeable changes in the chemical composition of the grass, in particular a sharp decrease in crude protein (CP) content (100-130 g/kg dry matter [DM]). During trial 1, milk protein content (+1 g/kg) and yield were higher under LN treatment. Conversely, milk, fat and protein yields were sharply reduced on LN sward in year 2. Pasture management, characterized by herbage allowance, probably induced different levels of energy supply that could account for these discrepancies. On the HN swards, the response to PDIE increase was linear, but restricted (+180 g milk and +3.5 g protein/100 g PDIE). On the LN swards, the response was greater and curvilinear. It probably reflected a level of metabolizable protein supply lower than the cows' requirements when PDIE concentrate supplementation was kept below 300 g/day. A supply of 600 g/day PDIE on LN swards produced animal performance similar to that obtained under the HN treatment without protected meal supplementation. These responses were mostly due to the improved protein nutrition of the animals and also of the rumen bacteria. However, the hypothesis of an indirect beneficial effect on grass intake should not be ruled out. According to these results, protein supplementation is not justified on correctly fertilized swards in the spring. PM addition to the concentrate diet of grazing dairy cows should be worked out in consideration of the consequences of nitrogen fertilization on the grass PDI (protein truly digestible in the intestine) content.