DAIRY - The Green, Green Grass of Home

1 May 2017

As the days lengthen and conditions start to warm up, the potential of utilising quality grazing into dairy rations becomes a welcome consideration again. Many regions of the world will indeed look upon our green isle at this time of year with some degree of envy because of our abundant ability to grow such a cheap high energy/high protein crop such as grass. 

However, although grazed grass does indeed have the potential to significantly reduce feed costs it does have limitations that we all need to be aware of, if we are to maximise utilisation of grazing without sacrificing yield and fertility.

What are the limitations of grazing dairy cows?

The two main limitations of grazing are grass dry matter content and grass availability. Surprisingly there is very little variation in grass ME through the season and it is in fact grass dry matter that has the biggest impact on cow energy intakes at grass.

Grass dry matter (DM) is of course wholly linked to weather conditions and can vary dramatically, from 10% DM during wet weather up to 20% during sunny conditions. Because the average cow can only physically eat up to ~75kg of fresh grass per day, such variation in grass DM has a significant effect on how much grass DM and consequently how much energy and protein she is actually consuming per day. For example, 75kg of grass at 10% DM provides 7.5kg of grass DM, which will only support two litres of milk. Conversely, 75kg of grass at 20% DM provides 15kg of grass DM and will support 20 litres of milk – meaning that on a sunny day cows will produce 10 times as much milk as during a wet day!

It is therefore important to have a realistic expectation of what level of milk grazing will be able to support, which on an average day at 15% DM is likely to be 15 litres at the most. This is ideal for low yielders, who with 5kg of compound through the parlour, will be able to produce 25 litres of cheap milk off of grass. It is however a different story for high yielders doing over 25 litres and very fresh cows with suppressed intakes post-calving who are likely to lose significant body condition if we don’t treat them differently.

How do we manage high yielding dairy cows at grass?

There are a number of feeding strategies that we can employ to support cows in early lactation when fresh grazing is available:

  1. House high yielders full time until they are pregnant, over 180 days in milk and under 30 litres when they can be turned out to grass full time.
  2. House high yielders during the evening on a full TMR to give the cows the opportunity to maintain reasonable levels of dry matter intake (DMI), especially during wet weather.
  3. Turn high yielders out to grass after first cut silage along with access to a buffer ration after morning milking. This limits the intake of wet low sugar grass in the morning, improves utilisation of better quality grass in the afternoon and means the cows will be hungry during their natural feeding frenzy up to the hours of dusk.

Monitoring Cow Performance at Grass

Whilst grazing enables you to produce cheaper milk, especially from low yielders, we still need to ensure that we provide these cows with a balanced ration that not only complements high protein, high sugar grasses but also provides sufficient long fibre to hold grass in the rumen long enough to fully utilise it. This is because although on paper fresh grass looks to provide more than enough energy, if it isn’t held in the rumen then we will lose a large percentage out of the back of the cow undigested. It is therefore just as important to measure feed efficiency at grass as it is during the winter months and knowing your rumen rate or the number kilos of milk produced for every kilo of feed DM consumed is an important figure to know and a useful  management tool to let you gauge how well your cows are doing and monitor the effect of any dietary changes that you make.

Rumen rate is automatically calculated as part of the Harbro Milk Monitor programme, which has been updated this summer with a grazing model that enables you to accurately predict how much fresh grass the cows are

likely to be eating by taking into account the grass DM and the number of hours they are out at grass. As well as allowing you to calculate the rumen rate of your cows, knowing how much grass the cows are likely to consume and the milk that it will support will also help you decide on the best summer feeding strategy to implement after taking into account the milk potential of your herd at this particular time of year.

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