The effect of low protein supplementation to dairy cows grazing clover grass during half of the day

The objective of this research was to study the effect on milk production and excretion of nitrogen of supplementing cows grazing clover grass for half a day with reduced protein. A low level protein supplement (11% crude protein in dry matter (DM)), which allowed for a balanced ration over 24 h, was compared to a normal level (17% crude protein in DM), which balanced the ration given while the animals were barn fed. One hundred and seventeen lactating Holstein cows in two commercial herds grazed on clover grass leys for a half day from May to June. The cows had limited access to low protein silage during the other half of the day and were fed one of the two levels of protein in the concentrate while in the barn. Average grass intake was 8.3 kg DM (19.8% crude protein in DM). Intake was estimated by calculating the difference between energy recommendations (for maintenance, growth and milk production and the energy value of feed intake in the barn). Low protein supplement significantly reduced milk yield (25.2 vs. 27.0 kg), fat (1077 vs. 1136 g) and protein (838 vs. 900 g) compared to the normal protein level. Fat and protein content were not affected. Daily milk urea concentration increased from 3.0 to 4.2 mM for low compared to normal protein levels in the concentrate. The cows fed low protein concentrate had a significantly lower milk urea content even after grazing forage with high protein content. The 33% highest yielding cows and the 33% of the cows in the earliest stage of lactation in the pre-experimental period were more sensitive to the lower level of protein in the diet than the other cows. The estimated excretion of nitrogen in manure per kg energy corrected milk (ECM) was lowest at low protein level. However, a potential for a decreasing nitrogen excretion without milk yield loss can be realized by allocation of concentrates with varied protein content, according to yield and stage of lactation.