By Lorna MacPherson, dairy consultant, SAC Consulting

With little rainfall this spring, cows that have been turned out early will have been grazing grass of high dry matter content. Grazing cows often look well at this time of year with glossy clean coats and may even have picked up a little condition with the high energy content of grass.

It is often difficult to know how much milk grass is supporting unless you are regularly monitoring grass growth in a paddock grazing system and testing for the nutritional quality of the grass.

During the recent dry spell of weather this spring, some grass analyses have been in the high 20’s and even early 30’s for dry matter percentage.

Recent UK average grass quality results from AHDB Dairy’s Forage to Knowledge programme has been 19.4% dry matter, 11.5ME and 21.8% protein (reported June 19).

Let’s assume in good dry weather, grass is 20% dry matter and 11.5ME and the target grass dry matter intake is 14kg (70kg fresh weight). On a very wet day, the dry matter of grass can be as low as 12%. With the same amount of grazing time, 70kg of grass is equivalent to only 8.4kg of dry matter.

A 650kg cow has a maintenance requirement of 75 megajoules (MJ) and to produce 1 litre of milk at 4%BF and 3.3%P requires 5.3MJ. Therefore, a cow producing 16 litres has a total requirement of 160MJ. Depending on the dry matter of grass, you can see there is a huge impact on the amount of milk that can be supported:

Scenario 1 Scenario 2

Grass fresh weight intake (kg) 70 70

Dry matter of grass (%) 20 12

Dry matter intake (kg) 14 8.4

ME intake (MJ) 161 96.6

Milk yield (litres) 16 4

A difference of 12 litres is equivalent to about 5kg of concentrate dry matter at 12.5MJ/kg DM. It is easy to see how cows can fall short of energy in wet weather and the impact that could have on body condition during a prolonged wet spell. On very wet days the energy deficit means that cows could easily be mobilising 1-2kg of body reserves.

Note that as the days start to shorten, grazing time declines and so expectations of how much milk grass can support must be reduced. Bear this in mind as the grazing season progresses.

By early July, M+16 litres can be achieved with good grazing management and ideal weather conditions. With lower grass intakes, first lactation heifers will yield around 3-4 litres less from grass.

Loss of condition is often not noticeable until later in the season, once the damage has been done. The knock-on effect is poorer fertility and is more of an issue for summer calvers. If cows fail to conceive and fall out of the desired calving pattern, the involuntary culling rate may increase. Poor energy status will also be reflected in low milk protein, which tends to indicate energy status of cows over the last 6-8 weeks.

If possible, split the herd and group early lactation, thin cows and those not in calf in a separate group for buffer feeding. Alternatively house these cows at night with access to more feed. At this time of year, with good grazing management, low yielders giving less than 25 litres can be maintained on grass and parlour cake alone.

Not only will this help maintain milk yields, protein and butterfat percentage should also benefit. Keeping the dry matter intake up and maintaining body condition should help improve cycling and conception rates and ensure cows enter the housing period in optimal condition to support winter milk production.