Getting the most from grass

19 April 2018


Oversupply of rumen degradable protein in grass

Typically, grazed grass is high in crude protein (CP), ranging from 15 – 25 %. However, in well managed pastures, particularly in the spring, a CP of 25 % and higher is becoming more common.

The 2017 GrassWatch study by Trouw Nutrition Ireland highlighted this, even recording instances of 30 % CP in grazed grass. Offering cows a low CP concentrate when they are grazing often does not result in reducing the CP in the total ration to 16 – 18 %. For example, if cows that are eating 15kg (dry matter) of 25% CP grass receive 7kg of a 12% CP concentrate, the overall diet will have a CP over 20 %.

In addition to grass having a high CP content, this protein is also highly degradable in the rumen (typically 70 – 80 % of CP). The high rate of protein breakdown in the rumen often exceeds the ability of the rumen microorganisms to ‘capture’ nitrogen (N) and synthesize microbial protein, because energy is a limiting factor. This means that rumen N is not utilized efficiently. Therefore, using concentrates that are high in rumen fermentable carbohydrates helps ensure that the rumen microbes can utilise as much of the protein as possible.  

Ensuring the efficient utilization of nitrogen by rumen microorganisms optimizes the supply of microbial/metabolisable protein to the small intestine for absorption, which is particularly important for higher yielding cows (achieving more than 27 litres) on full time grazing. In addition, enhancing protein utilisation will help to avoid conditions associated with excess rumen nitrogen.  

Excess rumen nitrogen at grazing, as a result of inadequate rumen fermentable carbohydrates to utilize the high total and rapidly fermentable protein, can often result in the following clinical signs:

  • Loose faeces
  • Reduced milk yield
  • Drop in milk fat composition
  • Body condition loss
  • Urine burns on pasture
  • Detrimental impact on fertility

Although no conclusive evidence has been demonstrated, studies tend to report negative associations between a number of fertility parameters and a high/more rumen degradable CP diet (also seen as high plasma urea and milk urea nitrogen). In Ireland, when cows go to full-time grazing, fertility tends to drop. As a result, some farmers (especially in the North of Ireland) do not let cows out to pasture until pregnancy is confirmed, otherwise they let cows out to full-time grazing and have to accept that fertility may ‘take a hit’.

Understanding nitrogen digestion in ruminants

In the rumen, protein is broken down to amino acids and then into ammonia (plus branched chain fatty acids) by the rumen microorganisms. In addition, non-protein nitrogen (NPN) is hydrolysed by ureases enzymes to ammonia. Rumen microbes then use this ammonia to synthesize their own amino acids for growth and replication (ultimately the main source of protein for the ruminant). The extent to which ammonia is used by the rumen microorganisms to synthesize microbial protein, is largely dependent on the availability of energy that is generated by rumen carbohydrate fermentation. On average, 100g of carbohydrate fermented in the rumen will produce 20g of microbial protein.

Diets that have highly soluble proteins or high proportions of NPN, combined with inadequate rumen fermentable carbohydrate, can cause a rapid production of ammonia, resulting in high rumen ammonia concentrations. Ammonia is then absorbed into the bloodstream and as ammonia is toxic, the liver converts it to urea. The urea concentration then rises in body fluids, including blood (blood urea nitrogen, or BUN), milk (milk urea nitrogen, or MUN), before excretion from the body through urine and milk.

The ‘cost’ of excess rumen ammonia

The process of absorbing the excess rumen ammonia and converting it to urea in the liver requires energy and is commonly referred to as the “urea cost”. The equation for calculating milk loss from excess nitrogen is shown below. However, a ‘rule of thumb’ is that the energy required to process 350g of excess rumen nitrogen is approximately equivalent to the loss of one litre of milk.  

Milk loss = (Excess N (from analysis) – 200) x 0.006

When examining Grass Watch data, there is a linear relationship between the crude protein and excess nitrogen in grass (Figure 1). Using this, the following can be concluded:

  • When feeding a 20% CP grass, this supplies 300g/day of excess  nitrogen, which corresponds to an energy loss equivalent of 0.6 litres of milk.
  • When feeding a 24% CP grass, this supplies 650g/day of excess nitrogen, which corresponds to an energy loss equivalent of 2.7 litres of milk.

Figure 1: The relationship between crude protein and excess nitrogen in grass during the GrassWatch 2016 study.


Using the Nutriopt NFEPB figure to manage the energy protein balance at grass

Nutriopt Fermentable Energy Protein Balance (NFEPB) is a useful tool to help manage the energy protein balance during grazing. The NFEPB figure is the difference between the available rumen-degradable protein and the rumen-degradable protein required for maximum microbial protein synthesis based on the available amount of fermentable carbohydrates. If the NFEPB figure is positive, the diet is high in protein, whilst if NFEPB is negative, the diet is high in fermentable carbohydrate. A perfectly balanced diet will have a NFEPB of 0g/day. The recommended NFEPB for a complete diet is between 0 and 200g/day, as higher figures will result in a significant waste of rumen fermentable protein.

The NFEPB value on Nutriopt reports for fresh grass will allow the composition of the concentrate supplementation at grass to be modified effectively so that the rumen works more efficiently. However with such a high rumen degradable protein content in grass it is extremely difficult to safely include enough rumen fermentable carbohydrate in the concentrate to sufficiently balance the rumen degradable protein content. Therefore calculating the NFEPB of the complete diet will assist in indicating if feed biotechnologies are needed to help deal with the remaining excess rumen ammonia.

Figure 2: The relationship between grass crude protein and NFEPB intake from grass (assuming grass DMI of 14kg) during the 2017 GrassWatch study

Using Figure 2, a 25 % crude protein grass results in NFEPB contribution from grass of 1200g/day. If a cow is eating 10kgDM of concentrates (with a typical NFEPB of -10g/kgDM), this will only result in the total dietary NFEPB being reduced by 100 to 1100g/day.


Milk urea records and NFEPB to manage the energy protein balance at grass

Bulk tank ‘milk urea’ (MU) or ‘milk urea nitrogen’ (MUN) records are seldom utilised on farm, despite them providing useful indications on how well the rumen is balanced for energy and protein. A high MU (or MUN) indicates an excess of rumen fermentable protein. A ‘rule of thumb’ is that MU is 19mg/dl (MUN is 8.9mg/dl) at 0g NFEPB/d, and MU increases by 2mg/dl (MUN increases by 0.94mg/dl) for every +100g NFEPB.

GrassWatch 2017 study recorded a seasonal average MU of 34mg/dl (MUN of 16mg/dl), which is equivalent to an average NFEPB of 750g/d for the complete diet at grazing (despite the average concentrate being a 14% CP). Given that the recommended NFEPB is 0 – 200g/d, this indicates a significant waste of rumen fermentable protein during grazing. This suggests that the current practice of offering a high energy low protein (12-14% CP) concentrate is not effective enough to balance the high CP in grass. Therefore additional methods such as rumen biotechnologies may play a role to decrease rumen protein degradation and/or increase nitrogen utilisation by rumen microorganisms.

(Note: MUN (mg/dl) = MU (mg/dl) x 0.47)

Table 1 - Bi-monthly averages for GrassWatch 2016/2017 protein efficiency parameters

Table 1 shows the GrassWatch 2016/17 bi-monthly average values for the parameters associated with protein utilisation efficiency. As the grazing season progressed, the grass NFEPB increased. This is because the ME and sugars decreased throughout the grazing season at a greater rate than CP. In some farms, CP actually rose again towards the end of the grazing season.


In summary, well-managed pastures will have excess rumen degradable protein relative to rumen fermentable carbohydrate, resulting in protein wastage.  The current strategy of feeding low protein high energy concentrates is insufficient to off-set this imbalance adequately. Using Nutriopt NFEPB and milk urea records can identify the extent of the imbalances by individual farm cases.