By Conrad Ferris
and Charlie Sullivan,
Livestock and Production Sciences Branch, AFBI Hillsborough
DAIRY cow diets contain nitrogen, mostly in the form of protein. However, nitrogen use efficiency by dairy cows is low, with only approximately 30 per cent of nitrogen that is consumed being converted into milk protein. Most of the remaining nitrogen is excreted in manure, and some of this can be lost to the atmosphere (as ammonia and nitrous oxide), and to waterways (as nitrates).
There is currently considerable pressure to reduce these losses of nitrogen from dairy systems within Northern Ireland. Recognising these challenges, DAERA, supported by co-funding from John Thompsons and Son Ltd and Trouw Nutrition Ltd, have commissioned AFBI to conduct a wide-ranging research programme to identify strategies to improve nitrogen-use-efficiency in dairy cows.
One component of this project has involved examining practices being implemented in a number of European countries, including the Netherlands.
‘Nitrogen crisis’ in the Netherlands
It has been said that the Netherlands is suffering a ‘nitrogen crisis’, with court action forcing the government to implement new and more stringent laws to reduce nitrogen emissions in the country by approximately 45 per cent. These new laws, enacted by the government in 2020, seek to reduce emissions across all parts of society, including transport, construction and agriculture.
However, as approximately half of all nitrogen emissions in the Netherlands arise from agriculture, the agricultural sector is under particular pressure.
Indeed, the agricultural sector in the Netherlands currently has one of the largest ‘nitrogen balances’ of any EU country (nitrogen balance refers to the difference between nitrogen added into an agricultural system, and the nitrogen removed from the agricultural system, per hectare of land).
While the new regulations may
lead to a contraction of the agricultural sector in the Nether-lands, farmers are also now addressing the problem head-on by adopting improved nutritional and management practices designed to limit emissions.
The benefits of reducing total diet crude protein concentrations are considerable, with Dutch research demonstrating that reducing total ration crude protein content by one per cent has the potential to reduce ammonia emissions by 10 per cent.
Progress is now being made. For example, in 2020 a typical ration offered to dairy cows in the Netherlands had a crude protein content of approximately 17.0 per cent (dry matter basis).
However, given the current pres-sures, a voluntary agreement has now been reached between the Dutch government and farming organisations to reduce total ration crude protein concentrations to 16.0 per cent by 2024. Discussions
with researchers and scientists have highlighted a num-ber of key issues from the Dutch experience.
n Do not overfeed protein. The Dutch rationing system is based on ‘Digestible protein in the small intestine’ (DVE), which is similar to the Metabolisable Protein system in the UK. Farmers are urged to supply the correct amount of DVE (or metabolieable protein) for the production level on their farm, and no more. In the past some extra protein (a ‘safety factor’) was fed ‘just to be sure’ there really was enough protein in the diet. However, the Dutch have now realised that the inclusion of even a small ‘safety factor’ of 2-3 per cent can increase total diet crude protein levels quite significantly.
n Ensure that protein and car-bohydrates are balanced in the rumen. In the Netherlands this is assessed as ‘Rumen-degraded protein balance’ (OEB). In the past it was recommended that value was positive, namely, to oversupply rumen degradable pro-
tein, compared to energy. However, if farmers are to reduce diet crude protein levels, there is an acceptance that this value needs to get close to zero (namely, no surplus rumen degradable protein).
n Feed more ‘glucogenic energy’. Glucogenic energy is derived from sugars, starch and digestible fibre. Feeding more glucogenic energy will increase microbial protein synthesis in the rumen, increase amino acid uptake from the small intestine, and increase the efficiency of protein metabolism. As crude protein level in the diet is reduced, it is thought that glucogenic energy supply must be increased. However, this must be done while avoiding rumen health problems such as sub-acute rumen acidosis (SARA).
n Accurate rationing of herds: If farmers are to begin to adopt lower protein diets, having regular accurate analysis of forages being offered is essential. Rations must then be developed that complement these forages. Modern feeding approaches (such as Feed-Into-Milk in the UK) which focus on ‘metabolisable protein’ (not crude protein) must be used. Farmers must increasingly move away from the use of ‘crude protein’ as a rationing metric.
n Inclusion of maize silage in the diet: Including maize silage in the diet (30-35 per cent of forage DM) can help farmers achieve these lower protein rations. This can be more challenging with grass/grass silage based diets, especially when protein levels are high.
While the national target in the Netherlands is a total diet crude protein content of 16 per cent, some research groups are going further and are actively seeking to achieve crude protein concentrations of 15 per cent DM in dairy cow diets on farms.
A number of farmers working under close supervision of nutritionists have achieved these low protein levels, while still maintaining high levels of performance. To reach even lower crude protein levels it may be necessary to supplement diets with methionine and lysine, the first-limiting amino acids.
However, in general both nutrit-ionists and researchers in the Netherlands indicated that there was a need for more consistent data on the response to specific amino acids before advocating their use. A number of scientists believe that to achieve very low diet protein levels, it may be necessary for farmers to accept a small loss in performance for the greater good of the whole industry.
Legislative pressure in the Neth-erlands has forced farmers to reconsider protein nutrition of their herds. Progress has been made, and total diet protein levels look set to be reduced, albeit the focus has long since moved away from ‘crude protein’ in ration formulation, focusing now on optimising the supply of ‘metabolisable protein’ to the small intestine.
Research at AFBI is currently building on approaches already adopted in the Netherlands and in other countries as we develop our own research programmes, and identify solutions that are appropriate for Northern Ireland to significantly reduce nitrogen excretion, and increase nitrogen use efficiency in dairy cows through dietary manipulation.
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