BRIEF COMMUNICATION: Can milk urea nitrogen differentiate nitrogen use efficiency of lactating cow groups raised on New Zealand pasture?

摘要

Improving nitrogen use efficiency (NUE) (g of milk nitrogen output/g of nitrogen intake) of dairy cows helps to maximise profitability and minimise nitrogen losses on farm. However, the traditional nitrogen balance technique used indoors to measure dry matter intake (DMI) and NUE is labour intensive, costly and unsuited to pasture-based grazing systems. In The Netherlands, milk urea nitrogen (MUN) has been used as a non-invasive and easy-to-conduct test to differentiate NUE of dairy cow groups (Kuipers et al. 1999). Generally, higher values of MUN are associated with a lower NUE. However, the usefulness of MUN in New Zealand pasture-based grazing systems is uncertain for numerous reasons. Firstly, MUN excretion reaches a maximum when dietary crude protein (CP) exceeds 20% of feed dry matter (DM), reducing the sensitivity of MUN to differentiate NUE in high dietary CP systems (Reynolds & Kristensen 2008). Secondly, water intake has been identified as one of the factors influencing the concentrationof MUN, but it is not normally quantified in grazing systems (De Campeneere et al. 2006). Lastly, when individual cow samples are used, MUN may be subject to diurnal variation (Cheng et al. 2010).