MILK UREA CONTENT ON SUPPLEMENTED GRAZING DAIRY COWS IN GALICIA - A. González Rodríguez and O.P. Vázquez Yáñez

Centro de Investigaciones Agrarias de Mabegondo, Galicia, Spain

SUMMARY

Milk urea nitrogen as an estimator of protein metabolism was measured in dairy cows grazing from late spring to summer ryegrass-clover pastures supplemented with different levels of crude protein (CP). Thirty-two dairy cows calving from February to March were divided into four groups. All the groups grazed a ryegrass-clover pasture from early June to mid August (11 weeks) and were supplemented concentrate depending on the treatment assigned to the group. The treatments were: no supplementation, 14 percent CP ration, 17 percent CP ration and 20 percent CP ration. Supplemented groups had a significantly higher milk production compared to no supplementation group. Milk protein was significantly higher for 17 percent CP treatment. Milk urea values were 244, 295, 317 and 364 mg urea/kg for no supplementation, 14 percent CP, 17 percent CP and 20 percent CP groups, those values were significantly different. Results showed the influence of the total CP content of the ration on milk urea content variation in grazing cows.

Keywords: test urea, milk protein, grazing cows, supplemented cows

INTRODUCTION

Protein metabolism has a major influence on milk production and nitrogen balance in grazing dairy systems. The level of crude protein (CP) in pastures is an indication of the potential milk yield in grazing cows. Higher levels of CP in pasture usually represents an increment in milk production when all the rest of the nutrient requirements are covered, but it has been observed that too high CP levels in the ration (higher than 19 percent) can produce a reduction on reproduction performance (Staples et al., 1992) and health problems. The response to different levels of CP is affected by the degradability of the CP in the rumen (Broderick, 1995) and its amino acid composition. Net energy (ENl) and non-fibre carbohydrates content of the ration are factors involved in CP metabolism (Broderick, 1995).

CP content and quality in pastures varies with the type, stage of maturity and fertilization of pasture. Usually pastures show a high level of CP during early spring and a high level of CP degradability in rumen (Hoffman et al., 1993). This could mean a lower protein utilization during this period of time.

Recent studies show that Milk Urea Nitrogen (MUN) could be a good indication of the utilization of the protein in dairy cows. MUN is highly correlated with blood urea nitrogen (Baker, 1995) which is a product of the CP metabolism. Several studies have shown that MUN is correlated with CP efficiency (kg of milk protein/kg of CP produced) and the ratio CP/ENl.

The objective of this experiment is to study the effect of different levels of CP in the MUN of supplemented cows, grazing ryegrass-white clover pastures in Galicia, to define adequate levels of MUN for grazing conditions.

MATERIALS AND METHODS

Thirty-two Friesian dairy cows calving from February to March were divided into four groups. All the groups rotationally grazed a ryegrass-white clover pasture from early June to mid August (11 weeks) in 1996. During this period, they were supplemented with concentrate depending on the treatment assigned to the group. The treatments were: no supplementation, 14 percent CP ration, 17 percent CP ration and 20 percent CP ration based on an average 12 percent CP pasture. Table 1 shows the concentrate composition and the nutrient ration composition during the experimental period. All the groups grazed the same pastures, supplemented groups consumed 5.9 kg DM/day of concentrate supplied twice a day after each milking.

Milk production was measured daily. Milk urea, fat and protein were sampled three times a week and analysed using Milkoscanã. MUN was measured as urea in milk (mg/kg).

Table 1. Feed and nutrient composition of concentrates consumed by supplemented groups.

RESULTS AND CONCLUSIONS

The results in milk production, milk protein, fat and urea are shown in Table 2. These results show that supplemented groups had a significantly higher milk production compared to the no supplementation group. These differences cannot be attributed only to CP level in the ration but also to total daily energy intake, which was lower for the non supplemented group. Milk production differences among supplemented groups were not significant, although the 14 percent CP group had lower average production than the other two groups with similar production. Milk protein was significantly higher for 17 percent CP treatment. Milk urea values were 244, 295, 317 and 364 mg urea/kg for no supplementation, 14 percent CP, 17 percent CP and 20 percent CP groups. No supplementation and 20 percent CP values were significantly different than 14 percent CP and 17 percent CP groups.

Table 2. Milk production, quality and milk urea for grazing rations with different CP content.

Figure 1. Milk production and milk urea for mixed sward grazing rations with different CP content.

These results show that using too high levels of CP in grazing rations does not improve total production significantly, so CP is not used in an efficient way. The excess of CP in the ration implies a higher urea content in milk. Several authors consider that levels higher than 19 percent CP in the ration could challenge health and reproduction (Staples et al., 1992). As 20 percent CP ration has a higher CP level than the previous, it could be concluded that 364 mg urea/kg is a level out of the optimum.

Results show the influence of the total CP content of the ration on milk urea content variation in grazing cows. Milk urea can be considered as an interesting tool for diagnosis for inadequate protein balance in rations for grazing cows.

REFERENCES

Broderick, G.A. 1995. Use of milk urea as an indicator of nitrogen utilization in the lactating dairy cow. In 1995 Research Summaries. U.S. Dairy Forage Research Center, Madison WI.

Hoffman, K., Muller, L.D., Fales, S.L. & Holden, L.A. 1993. Quality evaluation and concentrate supplementation of rotational pasture grazed by lactating cows. Journal Dairy Science, 76: 2651-2663.

Staples, C.R., Thatcher, W.W., Garcia-Bojalill, C.M. & Lucy, M.C. 1992. Nutritional influences on reproductive function. In “Large dairy herd management”. Edited H.H. Van Horn and C. J. Wilcox. American Dairy Science Association. Chanpaign IL 61820.