Grass and the ‘high-yielding’ dairy cow

Published 31 March 15

Dairy cows yielding over 40kg of milk (2.4kg fat + protein) per day are commonly fed a TMR with relatively little fresh pasture in the diet. But does this production system optimise profitability and what happens when TMR is substituted with zero-grazed grass, asks DairyCo R&D manager Dr Stephen Whelan?

As part of the DairyCo research partnership, researchers at SRUC conducted a study to find out just that. In a trial that began at the end of April 2014, cows yielding 38kg milk (2.3kg fat + protein) per day at the beginning of the study were divided into one of three feeding groups:

Group 1:              100% total mixed ration (TMR)

Group 2:              75% TMR and 25% fresh grass, on a dry matter (DM) basis

Group 3:              50% TMR and 50% fresh grass, on a DM basis

The TMR was formulated to meet the requirements of production. The fresh grass was cut and collected in the morning, analysed for DM and included in the ration at the expense of TMR on a kg-by-kg basis at the appropriate rates. This feeding strategy was employed for 16 weeks, during which time animal performance was measured. At the end of the feeding period the data was analysed and an economic analysis of the feeding strategies conducted to assess which of the feeding systems above delivered the greatest economic surplus and under what conditions.

In the study, the cows offered the 50:50 TMR: grass diet delivered the greatest economic surplus under a range of milk price and feed cost scenarios. However, the low DM (12-18%) of the grass offered during this study limited dry matter intake (DMI), particularly at the 50% inclusion level where DMI fell by 2kg per day (Table 1). This in turn resulted in a 4.3kg milk (0.29kg fat + protein) per day reduction in yield for cows offered the 50:50 diet versus those offered the 100% TMR diet.

Interestingly, the cows offered the 75% TMR-based diet delivered the lowest economic surplus regardless of milk or feed price. These cows had a similar DMI to those offered the 100% TMR diet but only produced the same milk yield as those in the 50:50 diet.

Table 1. Effect of TMR substitution with fresh grass on milk production, intake and economic surplus, at 25ppl milk price and a cost of £84 and £15 per tonne of TMR and grass, respectively

TMR to Grass Ratio

100: 0

75: 25

50: 50

Milk yield (kg/cow/day)




Milk Fat %




Milk Protein %




Milk solids (kg/cow/day)




Intake (kg DM/cow/day)




Economic surplus at 25ppl milk price (£/cow/day)




Dr Mark Lee, researcher at SRUC, explained that the costs for the TMR was £84 per tonne as fed, while the cost of including fresh grass in the diet was £15 per tonne as fed. This resulted in the 50:50 diet delivering the highest economic surplus per cow up to a milk price of 32ppl. However, once the milk price increased above 32ppl then the 100% TMR diet delivered the highest economic surplus. “We also conducted a sensitivity analysis of our results,” explains Dr Lee. “If TMR price was 10% lower for example, then the price at which this diet becomes more profitable than the 50:50 diet is 27ppl.”

As feed represents one of the largest costs in producing a litre of milk, incorporating cut and carried fresh grass into the diet of the ‘high-yielding’ dairy cow should allow for lower cost milk production. Farmers who are looking for options on including fresh grass in their ration need to assess the quality of their grass and work out the costs as they occur on their own farms, factoring in any milk yield penalty that may occur. However, with careful management, including grass in the diet of the dairy cow can be a success.

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