Reducing dietary crude protein levels while meeting metabolizable protein requirements: Performance of dairy cows over a full lactation period.

The nitrogen use efficiency (NUE; defined as N output in milk/N consumed) of dairy cows within pasture-based systems is rather low, often less than 25%. Excreted N, in particular the rather high urinary N, may thus be lost to the atmosphere as ammonia and nitrous oxide and to waterways as nitrates. Given the strong positive relationships between dietary N content and intake and urinary N excretion, lowering the crude protein (CP) content of the diet should improve NUE and reduce N loss to the environment. However, although reducing the CP content of dairy cow diets can lower feed costs, dairy farmer adoption will only be acceptable if accompanied by minimal or no loss in productivity, since milk yield per se has the highest correlation with profitability. The correct metabolizable protein (MP) content of the diet theoretically should limit N loss through urine and therefore may allow a decrease in the overall CP content. Thus, the study cited was designed to observe performance and nutrient utilization when cows were offered diets containing a range of CP concentrations, but with all diets designed to meet the cows’ MP requirements in early lactation. It was hypothesized that NUE in early lactation could be improved by reducing dietary CP levels, without loss in performance, provided that MP requirements were met.  

In the study, 90 Holstein dairy cows (24 first lactation, 66 multi-lactation) were fed diets containing either 150, 160, or 170 g CP/kg DM from 8 to 180 days in milk (DIM), with all diets designed to supply at least 100% MP requirements. On day 181, half of the cows in each treatment changed to a diet containing 140 g CP/kg DM (supplying 100% MP requirements), with the remaining cows continuing to be given their original treatment diets. This resulted in six treatments in the mid-late lactation period (181–280 DIM): 150, 150/140, 160, 160/140, 170, and 170/140 g CP/kg DM. Measurements will be clear from the results.

By decreasing dietary CP concentration from 170 to 150 g CP/kg DM did not have an effect on DMI, milk yield, milk fat or protein yield in early lactation (8– 181 DIM). It did, however, reduce MUN and the ratio of n-6 to n-3 fatty acids in milk, as well as serum albumin, globulin, total protein, and the urea concentration. In addition, by reducing the dietary CP content from 170 to 160 g CP/kg DM improved NUE, but with no further improvement with 150 g CP/kg DM. Treatment had no effect on apparent total-tract digestibility in early lactation. Urinary N output decreased with decreasing dietary CP content in early lactation, whereas faecal N plus urinary N output increased. The % urinary N in this total output decreased from the 170 to the 160 g CP/kg DM diet, whereas no further reduction was observed with the 150 g CP/kg DM diet. Cows that remained on the 150 g CP/ kg DM treatment in mid-late lactation (181–280 DIM), had lower DMI than those which remained on the diet containing 170 g CP/kg DM. When the dietary CP concentration was reduced to 140 g CP/kg DM in mid-late lactation, DMI, milk yield, and milk fat and protein yields were reduced compared with the 170 or 160 g CP/kg DM diet throughout lactation. Concentrations of C18:2 cis-9,trans-11 and the ratio of n-6 to n-3 fatty acids in milk were lower for cows offered diets containing 140 or 150 g CP/kg DM in mid-late lactation compared with 160 or 170 g CP/kg DM. By reducing the dietary CP concentration from 170 to 140 g CP/kg DM NUE was improved from 0.28 to 0.34 in mid-late lactation. Nitrogen digestibility was reduced when cows were offered the 140 g CP/kg DM diet compared with the 150 or 160 g CP/kg DM diet, and by lowering the dietary CP concentration from 170 to 140 g CP/kg DM in mid-late lactation, N output in milk, faeces and urine was reduced.

In conclusion: The results indicate that a dietary CP content of 160 g CP/kg DM, which met the MP requirement of cows, may be optimal to support performance over a whole lactation, improve NUE, and reduce N excretion, thus contributing to a more sustainable approach to dairy cow production.