Monitoring CP usage in dairy cattle rations by using milk urea as indicator in a nitrate vulnerable area

Milk urea concentration (MU) has been used as an indicator of nitrogen metabolism in ruminants. In particular, linear relationships between MU and protein percentage in diet (CP) or nitrogen excretion have been reported. Thus, the objective of this research was to study the dietary protein utilization of lactating dairy cows of the area of Arborea (Oristano, Sardinia, Italy), recently declared vulnerable to nitrates, by using MU as an indicator. The average milk yield in the province of Oristano is the highest in Italy. The area of study includes 5500 ha of this province, with about 16000 dairy cows, distributed in 163 dairy farms. A database (1733 monthly controls in 155 farms with 86±52 heads per farms) for this area was created by using the information recorded by the local dairy herd organization (APA; Associazione Provinciale Allevatori di Oristano, Italy). In particular, aggregate mean monthly records per each farm in the year 2006 on milk production, MU and milk protein concentration were considered. August data were collected in 35 farms only. The mean optimal MU value of 23.5 mg/dl was calculated based on the average of the values 10.3 and 11.7 mg/dl of milk urea nitrogen proposed, for zero predicted surplus of N, by Hof et al. (1997) and Nousiainen et al. (2004), respectively. A reference optimal MU range of 21 to 26 mg/dl was defined assuming a physiological variation of 2.5 mg/dl around this mean. Thus, MU values lower or higher than the reference range were considered an indication of diets with deficient and excessive protein supply, respectively, possibly associated to low N utilization efficiency for milk production. High seasonal variation and significant differences between farms were observed. Milk production in the area of study for the year 2006 was 30.6±3.6 kg/d per cow, with the highest production in the last part of the winter and the lowest in summer. Milk protein content was 3.31±0.12%, being lower in the summer than in the other seasons. MU was on average 22.7±4.2 mg/dl, with a peak in July. Of the 155 farms considered, 13% had a year mean of MU <21 mg/dl, 9.3% had it �26 mg/dl, the rest was within the optimal range of 21-26 mg/dl. None of the farms had a yearly mean MU above 30 mg/dl. On the basis of the yearly farms means of MU, in the area of Arborea a mean dietary CP of 14.7% of DM (range 13.9%-15.6% of CP, DM basis) was estimated on the basis of the relationship: CP (g/kg of DM)=99.8+4.5xMU(mg/dl)/2.144 suggested by Nousiainen et al. (2004). Considering the monthly MU means for each farm, for 25% of the months the farms had MU<21 mg/dl and for 20% of the months the farms had MU �26 mg/dl. Within each season the highest milk yield occurred when MU was between 22 and 23 mg/dl. In conclusion, the utilization of MU as a nutritional indicator of dietary CP concentration suggested that in the farms of Arborea the possibility of reducing N excretion of lactating dairy cows by nutritional means is very limited. Indeed, the results indicate that for 25% of the months there was probably a shortage of CP in the diets. The Authors would like to thank the APA of Oristano for providing the data used for the research. Ital.J.anIm.ScI. vol. 8 (Suppl. 2), 2009 346 02_NUTRITION. indd 346 20-05-2009 11:31:57 Effects of pelleting on rumen fermentability of different commercial concentrate mixtures Paolo Bani, Giorgia Astorri, Matteo Guerreschi, Valentina Ficuciello Istituto di Zootecnica, Università Cattolica del Sacro Cuore, Piacenza, Italy Corresponding author: Paolo Bani. Istituto di Zootecnica, Facoltà di Agraria, Università Cattolica del Sacro Cuore. Via Emilia Parmense 84, 29100 Piacenza, Italy – Tel. +39 0523 599279 – Fax: +39 0523 599276 – Email: paolo.bani@unicatt.it