Historically, dairy rations have generally formulated crude protein (CP) to the standard value needed to supply adequate feed nitrogen (N) in the cow’s diet, to help maintain the cow’s health, welfare and milk productivity.
In most diets CP is fed at an average of 16.5% of dry matter content (DM). Where these levels are exceeded, N emissions are found to increase, whilst milk production levels remain relatively static (Wonfor, 2017). Nitrogen use efficiency (NUE) in cattle is inherently poor, with approximately 75% of that consumed lost through a combination of faecal and urine output, along with body losses (See Figure 1). Of the N lost through excretion, that from urine is the most volatile and easily converted from ammonia into the greenhouse gas of nitrous oxide; 298 times more potent than carbon dioxide (EPA, 2017).
It is important to note that the type of dietary CP fed to cattle also influences NUE. Dietary protein is split into two main classes: rumen-degradable protein (RDP) and rumen-undegradable protein (RUP). The over estimation of required RDP, along with the inadequate provision of readily fermentable energy leads to higher urine N losses. However, it is essential that a cow’s diet contains adequate RDP to maintain effective fermentation
of dietary fibre, and ensure the delivery of microbial proteins to the small intestine. When looking to reduce dietary CP and RDP with the aim of increasing NUE, it is therefore important
to consider the sources of dietary energy and protein to prevent adverse effects to milk yields. By reducing surplus dietary protein, the cow’s NUE is increased and excreted losses decreased along with feed costs.
Several studies have looked at reducing CP levels in dairy diets, along with the effects on milk production. Of the studies conducted, results have varied in relation to milk production changes, with one study by Aschemann et al. (2012) finding no effect to milk yield when reducing CP to 12%, yet another by Olmos Colmenero and Broderick finding a loss of 1.98L/ day with a CP decrease from 16.5% to 13.5%. Another, by Varga (2007) found the opposite to Colmenero and Broderick, with an increase of 2.72L/day per cow and improved milk components as dietary CP was lowered from 18% to 16%.
Whilst there are differences between study findings, it can be concluded that in a well formulated diet, CP and RDP can be reduced without adversely impacting milk yield.
The reduction of surplus dietary CP not only improves the cow’s NUE, but also has environmental benefits. In the US, Hristov et al. (2011) found that the reduction of CP to 14.1% in concentrate fed to grazed dairy cattle only caused a slight reduction in milk yield losses. Similarly, in the UK, cattle grazed on high quality pasture and fed concentrates with a CP level of 14.1% saw negligible yield differences (Hynes et al., 2016). In both systems it was found that the lower CP diet level shifted the N excreted from urine to faeces, a much less volatile compound, thus reducing nitrous oxide emissions. A long-term project at Aber IBERS that is currently still underway has also found no substantial effect to milk yield from a lower CP based diet (Wonfor, 2017). Overall, the reduction of surplus protein in dairy diets provides benefits to the cow, the farmer and the environment.
Low CP diets can be made increasingly effective, with productivity enhanced when combined with a balanced supply of amino acids
Dairy Technical Specialist Coordinator
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