Abstract
Three experiments were conducted to investigate milk urea nitrogen concentration as a tool to monitor dietary protein status of dairy cows. In 1ˢᵗ experiment, a total of 100 dairy farmers, 50 dairy farmers in each region and season were selected to know the feeding systems of cows and chemical composition of feed stuffs. In 2ⁿᵈ experiment, a total 160 cows, 10 cows in each of native (local cow) and crossbred (local × crossbred) origins (genotype) differing in lactation yield were used in 2×2×2×2 factorial experiment using RBD to evaluate milk urea nitrogen of cows considering regions and seasons. In 3ʳᵈ experiment, 15 early lactating multiparous cows similar in live weight and lactation yield were randomly allocated to three treatments in a completely randomize design (CRD) to determine optimum feeding levels of rumen degradable protein to optimize milk urea nitrogen content. Three isocaloric diets differed in rumen degradable protein were formulated. The diets were not made iso-nitrogenous and variations and total dietary crude protein content remained constant level of energy. A control diet was formulated as per maintenance and production requirement. With constant ME concentration, the rumen degradable protein of the control diet was decreased or increased by 25% of the required level. Feeding practices followed by farmers in different feed bases and seasons were based on rice straw, small quantity of green grasses from various sources with small supplementation of concentrates. Milk urea nitrogen concentration positively correlated with intake of dietary CP and rumen degradable protein. Milk urea nitrogen concentration was higher under good base (38.86 mg/dl) than poor feed base condition (28.55 mg/dl). Season did not affect milk urea nitrogen concentration. Milk urea nitrogen of the local and crossbred cow was found 31.97 and 35.44 mg/dl, respectively. The stage of lactation did affect the level of milk urea nitrogen. Milk urea nitrogen was lower below 100 days in milk (31.48 mg/dl) than above 100 days in milk (35.93 mg/dl). Rumen degradable protein level in the diet increased, the efficiency of converting absorbed N to milk N was decreased substantially, and the lowest rumen degradable protein diet being most efficient (65.1%) in converting feed protein into milk protein. Urinary N excretion was positively increased or decreased with the increase or decreased of dietary rumen degradable protein. There was a positive correlation between daily rumen degradable protein intake and total purine derivatives excretion and microbial N yield. Purine derivatives excretion by 15 crossbred dairy cows (live weight: 332.4 ± 40.55 kg) was 114.87 mmol/d and calculated microbial N yield ranged from 65.5 to 79.9 g/d and from 21.4 to 23.8 g/kg of DOMR. Rumen degradable protein levels had no effect on milk yield (6.55 kg/d) and milk protein contents (3.73%). Milk protein was positively correlated with milk yield. Milk urea nitrogen and blood urea nitrogen increased linearly from 16.79 to 22.46 mg/dl and 19.07 to 25.91 mg/dl, respectively as dietary rumen degradable protein was increased from the lowest (-25%RDP) to the highest (+25%RDP) levels. Urea concentration was 16.97 % higher in +25%RDP and 12.55 % lower in -25%RDP compared to control diet (19.20 mg/dl). The observed values of milk urea nitrogen were higher than expected (12 mg/dl) based on previous work. However, the results reported herein suggest that an milk urea nitrogen concentration of 12 mg/dl may be inadequate to support maximum production under some conditions because cows on the 6.45% rumen degradable protein diet (+25%RDP) had a mean concentration of 22.46 mg/dl. Urea in milk and blood was closely correlated. Intake of CP and rumen degradable protein was positively correlated with milk urea nitrogen. Milk urea nitrogen was increased or decreased with increased or decreased of rumen degradable protein intake. It may be concluded that the plane of nutrition as a result of variation in feed availability may affect milk urea nitrogen concentration of dairy cows. It was observed that on-farm diets having variations in CP or rumen degradable protein affect milk urea nitrogen content of milk and this was confirmed by the on-station feeding trial. It was found from the trial that dietary CP and rumen degradable protein had positive impact on milk yield, milk composition and milk urea nitrogen. It was found that feeding dietary rumen degradable protein up to a level of 789 g/d had a significantly positive impact on milk urea nitrogen level, but diets having required level of rumen degradable protein for feeding dairy cows in local condition need to be confirmed through further research. The overall conclusion from this research is that milk urea nitrogen could be used as a tool to monitor for dietary nutrition and protein utilization of dairy cows.