The eternal question: 'How much milk can you get from grass?'

Published 5 June 15

Every year, many figures are quoted at meetings and in the press about maintenance plus X, Y or Z litres from grazing. The vast majority are, at best, on the conservative side, says Piers Badnell, AHDB Dairy Technical Extension Officer.

When I am asked the question: “How much milk can I get from my grass?” my answer is: unless you provide me with all the details, ie the size of your cows, the amount of litres they produce, what the fat and protein content of the milk is, what the quality of your grazing is and how much the cows are eating, I have no idea how much milk you can get from grass. It therefore amazes me that figures on this issue are put out as a standard, as it depends on the individual circumstances on your farm.  

It is very important not to over or underestimate grass dry matter intake, as this costs you money. Overestimating by loss of potential yield and body condition, and underestimating by expensive substitution and buffer feeding, which is not only expensive but it also does not teach the cows to graze properly.

There is a further cost to buffer feeding, the actual putting it out in terms of time, diesel, wear and tear on machinery, plus the cost of extra scraping yards and slurry disposal. Finally, the longer a cow is away from grazing, the less grass she can eat. A cow can bite 60 times a minute so, by reducing her time at grass, you reduce her dry matter intake. Grazed grass, as shown by Forage for Knowledge for the past seven years, if grazed at 2.5–3 leaves is 12 ME, so why would you reduce her intake?

How can you calculate the value of grass? For this, you need to know what she requires in terms of energy. As energy is the limiting factor it is an important calculation.

The information below comes from Chapter 7 of Feeding+, which is available for download from the AHDB Dairy website


PB Table

Rule of thumb

Using the data above, a 650kg cow requires 650 x 0.1 = 65 +10 = 75 MJ ME for maintenance. In terms of milk production, if, as in the example, she produces 3.8% fat and 3.2% protein, then for every litre she requires 5.2 MJ ME.

The table below shows what this data reveals about milk from grazing for this cow.

PB Table 2

Note: On a herd basis 17–18kg DM is about as much as a grazing cow can eat in a day.

The important points to get right are:

-          To manage grass well on a rotational basis

-          To get 12 ME all year

-          To measure it with a plate meter, so you know how much they are is eating

-          Knowing how heavy your cows are (without forgetting a proportion of your herd will be heifers and thus lighter, but on balance it is better to over feed energy than under feed especially for a growing heifer)

-          The amount of fat and protein they are producing.

A number of years ago, SRUC did a trial on dry matter intake at grass in the autumn on late lactation cows. It was found that intake on dry days was 15kg DM and on wet days this was 8–10kg DM. The trial was conducted late in the autumn but is a good guide to use. If there is ever a time the Holstein Friesian-type cow can really drop condition it is a wet autumn, so back to the don’t over or under estimate DM – measure, calculate and get it right.

Buffer feeding is for when you are short of grazing, and supplementation is for adding energy to sustain yield, and/or to fill a small hole in grass supply. Supplementation tends to be concentrate, which has a far lower substitution rate than conserved forage.

The cost of grass is in the region of 6p kg DM, silage is double that cost, and concentrate is double again. The more grass in the diet the cheaper and more profitable it will be. Do calculate your own grass and silage cost, as this is a guide, but the relative values are the important point.

The true production from grazing should be based on your farm data, not someone else’s guestimate. You need a plate meter to measure grass, it does the same job as the weighing scales on the feeder wagon in the winter.