Unlocking lower protein levels in the diets of early lactation dairy cow - precision formulation is key!

Date published: 05 October 2022

Area of Expertise:

Two of Northern Ireland’s leading animal nutrition companies, John Thompsons and Sons Ltd and Trouw Nutrition Ltd, are partnering with the dairy research team at AFBI to develop strategies to reduce nitrogen losses from dairy systems.

Dr Anna Lavery (left) and Dr Aimee Craig (right) from the Dairy Research Team, AFBI


Nitrogen, which is derived from the diet (mostly from protein), is an essential nutrient for dairy cows. However, dairy cows do not use nitrogen efficiently, and only approximately 30% of nitrogen consumed is converted into milk protein. Much of the remaining nitrogen is excreted by the cow in manure, and some of this manure nitrogen can be lost to the environment (as nitrates to waterways/ground water sources, and as ammonia and nitrous oxide to the atmosphere).

The efficiency with which nitrogen is utilised can be improved by a number of methods, including reducing the protein content of the cow’s diets. However, as protein is an important driver of milk production, there is concern that reducing protein levels in the diet could reduce performance. To address this issue, AFBI are undertaking a number of studies to improve our understanding of precision formulated lower protein diets on dairy cow performance. This project is being co-funded by DAERA, John Thompsons and Sons Ltd and Trouw Nutrition Ltd.

Overview of first study

The first study within this project involved 90 dairy cows and was conducted over the first 180 days of lactation. While these cows were offered diets which contained either 15, 16 or 17% crude protein, more importantly, all three diets were designed to meet the metabolisable protein requirements of the cows. This is because cows do not have a requirement for crude protein, but instead have a requirement for metabolisable protein. Metabolisable protein refers to the amount of protein (from either microbial protein or rumen undegraded protein) that is absorbed from the small intestine, to provide the amino acids that are necessary for milk production, maintenance, growth and pregnancy.

Within the current study all diets were based on a mixture of grass silage and concentrates (approximately 50 : 50 DM ratio), plus approximately 0.3 kg straw per cow per day. The same grass silage was offered with all diets, and this silage had a dry matter of 35%, a crude protein of 13.5%, and a metabolisable energy content of 11.6 MJ/kg DM. However, the concentrates offered with each treatment were formulated using different raw material inclusion levels to allow the different diet protein levels to be achieved. 


Total diet crude protein level had no effect on dry matter intakes, milk yield, milk fat and protein content and milk fat plus protein yield (Table 1).

  Total diet crude protein level
(% DM)
  15% 16% 17%
Total dry matter intake (kg/day) 23.2 23.3 23.8
Milk yield (kg/day) 35.7 37.1 36.3
Milk fat (%) 4.49 4.46 4.47
Milk protein (%) 3.44 3.48 3.49
Fat plus protein yield (kg/day) 2.82 2.92 2.89
Milk urea nitrogen (mg/kg) 97 115 134
Body weight (kg) 632 636 651
Body condition score 2.4 2.4 2.5

However, Figure 1 suggests that for cows on the 15% crude protein treatment, the yield of fat plus protein began to lag behind the yields with the other two treatments towards the end of the study period.

Figure 1: Effect of diet crude protein level on milk fat plus protein yield over the first 25 weeks of lactation
Figure 1: Effect of diet crude protein level on milk fat plus protein yield over the first 25 weeks of lactation
As expected, milk urea nitrogen content increased as diet crude protein level increased. This reflects the fact that when there is excess protein in the diet, some of the excess nitrogen is excreted in milk as urea. Cow body weight was unaffected by treatment, but cows offered the 17% crude protein diet had a higher body condition score. Reducing the crude protein percentage of the diet improved nitrogen use efficiency (calculated as nitrogen produced in milk, as a proportion of total nitrogen intake), with the 15 and 16% CP treatments having an efficiency which was approximately 9% higher than that of the 17% crude protein treatment (Figure 2).

Figure 2: Effect of diet crude protein level on nitrogen-use-efficiency (NUE)
Figure 2: Effect of diet crude protein level on nitrogen-use-efficiency (NUE)


The results of this study have a number of practical implications. Reducing total diet crude protein improved nitrogen use efficiency, and this will have reduced nitrogen loss to the environment. While reducing total diet crude protein to 15% had no statistical effect on milk and solids production, there was some evidence that this treatment may have been borderline in terms of supplying the requirements of the cows (as evidenced by the drop in fat plus protein yield towards the end of the study).

In conclusion, diets with crude protein levels of between 15 and 16% are likely to be adequate for dairy cows in early lactation, provided the rations are carefully designed and meet the metabolisable protein requirements of the cows. The impact of offering these lower protein diets on ammonia emissions is currently being assessed within the project.

Notes to editors: 

AFBI is an arms-length body of DAERA delivering research and development, diagnostic and analytical testing, emergency response capability and expert scientific advice for DAERA and other government departments, public bodies and commercial companies in Northern Ireland, and further afield.

AFBI’s Vision is “Advancing the Local and Global Agri-Food Sectors Through Scientific Excellence”.

AFBI’s core areas:

  • Leading improvements in the agri-food industry;
  • Protecting animal, plant and human health;
  • Enhancing the natural and marine environment.

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