Why has grass quality dropped?

Published 20 June 14

Grass quality 2014

Why have ME results dropped recently for many of our Forage for Knowledge contributors? Piers Badnell, DairyCo technical extension officer, offers some theories on the reason behind this drop.

Since the end of April, ME results from samples from many of our Forage for Knowledge farmer contributors,have dropped below 12 ME and are going against the trend we’ve seen in the last five years.

From previous data collected in Forage for Knowledge, we would expect ME to be 0.5 to 1 ME higher than it currently is. With a dry matter (DM) intake of 15kg DM/cow, at 4% fat and 3.3% protein, the drop of 1 ME is equivalent to 2.8 litres, a significant reduction.

Some caution has to be taken with these results, as the analysis does have some variation. Nonetheless, you might see the effects of this drop in cow performance. Are your cows actually showing this reduction in terms of: production, yield and constituents, and/or body condition?

This drop has also been seen in crude protein results. However, this is less important as it does not limit the cow, but shows a potential drop in pasture quality.

Output, in terms of production, depends upon two factors – ME, ie quality, and DM intake, ie how much is eaten. A reduction in ME can, potentially, be compensated by an increase in DM intake. This is illustrated in the table below.

Table 1: The effect of grass DM intake and ME value on potential milk output (litres/cow/day)

Grass ME

Grass DM intake

Supported milk yield










The formula to calculate the supported milk yield is: Grass ME x Grass DM intake – 70MJ, for maintenance, / 5.3MJ, requirement per litre.  

It is not so easy to establish the reasons for the drop in ME. Below are theories and suggestions of what might be causing the reduction.

At the moment, the grass plant seems to have more stem and, as such, it has lower quality. In terms of analysis, when comparing sampling weeks 23 May 2014and 6 June 2014 with similar weeks last year (21 May 2013 and 4 June 2013) a 13% increase in the NDF of the samples can be seen (42.9 in 2014, as opposed to 37.35 in 2013). This suggests more mature fibre and, as such, a reduction in quality.

When looking for a reason for the increase in stem, and this is my educated guess, we need to look at the wet spring many experienced. The wet weather early in the season potentially made it harder to hit residuals, and produced a higher than normal pasture rejection, as reported by some Forage for Knowledge samplers. This has meant a higher stem to leaf ratio, and a subsequent reduction in quality.

If these issues are not corrected in the second round of grazing, the problem will multiply, with a resulting ongoing effect on quality.

It is possible the grass plant is under stress and, as such, it wants to get a seed head up to reproduce (stress on the grass plant leads to heading). This increases the amount of stem the plant produces, and more heads on the plant means lower quality.

However, has the grass plant been under stress? We have had reasonably warm temperatures this spring (unlike the spring of 2013) and good soil moisture levels, so there are no obvious stress factors.

Whatever the cause of more stem on the grass plant, we now have to look at correcting this situation. Heading is virtually over. So, reinstate the residual, get rid of rejection sites and rejuvenate sward by pre-mowing and wilting, a silage cut or hitting hard with stock possibly young or dry stock. 

Another possible reason for the drop in quality has been put forward by Chris Duller, independent grassland and soil consultant. He suggests it is as a result of the extremely wet conditions we had in 2012. During this time the swards really took a hammering and bare patches probably increased as a result of poaching.

These bare patches are ideal to invade by grass weed species, such as meadow grass. These take one to two years to establish, so it’s not been until this season that we’ve seen an effect on pasture quality.  J Frame (1991) showed significant differences in the DMOD (D-Value) of grass species – perennial ryegrass 73 D value, as opposed to meadow grasses of 61 to 64 D.

Have a look at your sward. What is the proportion of ryegrass? Look for the red base of the sward and the keel shape to help determine how much ryegrass you have in the sward (see picture 1). If levels have fallen below 50% think about replacing it.

Picture 1 The red base of the ryegrass plant.

Ryegrass red bottom

In essence, I am not absolutely sure what the reason for the drop in quality is. However, there is an opportunity to try and correct this by reinstating a good residual in order to reboot the quality of the grass plant.