The objectives of the present study were to investigate phenotypic correlations of milk urea (MU) and efficiency of crude protein utilization (ECPU) with milk production and cow parameters in two contrasting herds. Between July 2016 and May 2017, 210 cows were milked twice daily with high supplementary feed inclusion (HS) and 258 cows were milked once daily with low supplementary feed inclusion (LS). In early, mid and late lactation, milk samples were collected to measure MU. At every herdtest date, ECPU was calculated as protein yield (PY) divided by crude protein intake (CPI); this last variable derived from intake estimations of metabolizable energy requirements. Positive correlations between milk yield (MY) and both dry matter intake (DMI) and CPI were observed in LS and HS. The ECPU only correlated positively with MY traits in LS, and there was no correlation of CPI with MU in either herd. A moderate negative correlation of ECPU with live weight (LW) was observed in both herds, but it was stronger in LS. By suppressing MY due to the reduced milking frequency of cows fed LS, these cows gained LW, had higher body condition score and CPI, and lower PY, reducing the ECPU. No correlation between ECPU and MU was detected.
New Zealand Journal of Animal Science and Production, Volume 78, Lincoln, 76-81, 2018
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