The role of grass in modern milk production

The ruminant has a unique ability to thrive on a multitude of diets, ranging from 100% grass and conserved forage to only arable by-products and concentrates. However, grass (usually grazed) is the major forage for dairy herds in much of Northern Europe and many argue that as it’s often the cheapest source of feed, so that its potential should be maximised. But it is also accepted that there are limitations to grass for higher yielding dairy cows, largely because of reduced control and consistency of feeding.

The aim of this blog is to discuss the role of grass for the modern high yielding dairy cow and to give some practical recommendations on grass feeding.

Grass composition

One of the difficulties when relying on grass is that its composition varies widely with season, species and management. This can be seen from the data summarised in table 1, based on 244 samples of fresh grass. Dry matter (DM) contents ranged from 11-42% of grass fre4sh weight with crude proteins (as % DM) between 5 and 36%, sugars 2 to 28% and NDF levels of 41 to 76%. It must be noted that this data includes primary spring growths, as well as summer and autumn re-growths harvested at different stages of maturity. There it may not be wholly representative of well-managed grazed grass, but this can still be of highly variable composition, often making it difficult to manage in respect of optimising feed utilisation.

Although grass composition can vary widely, there are important trends related to grass maturity. Young immature grass comprises mainly of leaf with little if any stem, so that cell contents typically make up <60% total dry matter, with cell walls (measured as neutral detergent fibre or NDF) often about 40%. Crude protein content is also typically high at this time at approximately 25% in dry matter, whilst sugar content is often low at about 10% in DM. However in mature (headed) grass, cell wall content (NDF%) increases to over 60% and cell content decrease to 40% or less. Crude protein content typically drops to <10% in DM whilst sugars can increase to 20% in DM, although this will depend upon weather conditions, especially the incidence of sunshine.

From this it follows, that well-managed, leafy grazed grass would be expected to be of high digestibility with a high metabolisable energy (ME) content, along with low fibre and high crude protein contents. However, there are known to be seasonal difference. In vivo studies have shown that the highest quality grass (12MJ/kgDM) only occurs in spring, when protein levels of about 25% are also common.

However, metabolisable energy contents of 11-11.5MJ/kgDM would probably be more typical of good grazing during the summer, and often with more moderate protein levels (18-20%). In the autumn, leafy regrowth grass is likely to have an ME content of between 10.5 and 11.0MJ/kgDM, often with relatively high protein levels (20 %+), although an increasing proportion of this may be present as non-protein nitrogen. There is substantial evidence that the efficiency of utilisation of ME is reduced in autumn- compared with spring-grass. This is likely to be a significant cause of the reduced levels of milk production typically observed on late summer/autumn grass compared with spring grass, though as suggested later other factors may be involved.

Author: Denis Dreux