Mowers, silage harvesters, balers and tractors and trailers have been on the move earlier than usual this year. Despite this, the majority of grass silage for the winter period ahead will be harvested in May and June.

By now, silage paddocks and fields should have received fertiliser between late March and early April and be closed for harvesting.

A recent report by Teagasc’s Eddie O’Riordan and Mark McGee in Grange indicates the impact of digestibility of grass silage for beef production.

While a number of factors can affect pasture yield and quality, harvesting date is one of the main influences determining the feeding potential of the resulting grass silage.

The pattern of change in grass yield on silage swards over time is depicted in figure 1 (blue line). It shows a steady increase in yield throughout the month of May and a slower rate of accumulation in June.

The potential nutritive value of the crop slowly declines during the first three weeks of May, but typically stays above 75% DMD (dry matter digestibility).

However, later in the month (depending on sward age and grass varieties sown), the pasture begins to produce stems and seed heads.

Figure 1: Pattern of change in grass yield and digestibility. Source: Teagasc, Grange Beef Research Centre

As a result of the changing plant structure, its nutritive and feeding value declines considerably. By late June DMD can be less than 60%.

Depending on the animal type(s) to be fed and their target performance level over the winter, a decision relating to harvest date needs to be made.

Data from Grange (below) shows that as DMD increases, animal dry matter intake (DMI), liveweight gain, carcass gain and feed conversion efficiency all improve.

At a DMD of 70% or greater, finishing animals can achieve a liveweight gain in excess of 0.66kg/day on silage only.

Digestibility: What difference does it make for beef cattle? Source: Teagasc, Grange