Dr Eddie O’Riordan made a presentation on the final night of the Teagasc beef conference on beef production from grass-forage diets only.

This is one of the studies currently been undertaken at Teagasc beef research centre in Grange, Co Meath.

Eddie firstly outlined the diet proportions of a typical steer and heifer production system where steers are finished at 25 months and heifers at 22 months. Typically in these systems, when we look at the diet on a dry matter (DM) basis, two-thirds of the lifetime diet of steers comes from grazed grass, while this increases to 70% in the case of heifer production. Concentrates on a DM basis only make up 14% and 10% of steer and heifer lifetime diets, respectively, with the remainder coming from grass silage.

When we look at diets on a fresh-weight basis, the lifetime proportion coming from concentrates is even lower at just 3% for steers and 2% for heifers. This is because concentrates have a much higher dry matter than both grass and grass silage. This is noteworthy as in the recently published Grass-Fed Standard for beef from Bord Bia, animals must have at least 90% of the diet coming from forage on a fresh-weight basis.

Pre-grazing yield

The next study outlined on the night compared two pre-grazing swards heights of either 1,500kgDM/ha or 2,000kgDM/ha over an entire grazing season for yearling suckler-bred steers. The animals that grazed the 2,000kgDM/ha pastures ended up 12kg-14kg heavier than those grazing the lower pre-grazing yield.

This weight difference was carried right through to slaughter the following spring when the animals that had the higher pre-grazing yield through the grazing season had a 5kg heavier carcase than the other group of cattle.

The conclusion of this study was that lower pre-grazing yields restricted liveweight gain in beef cattle.

Post-grazing sward height

Another study looking at the effect of post-grazing sward height on animal performance compared a 4cm and 6cm post-grazing sward height. Animals grazing to 6cm had a greater daily liveweight gain to the tune of 0.1kg/day. This resulted in animals being 24kg heavier at the end of the grazing season compared to those animals that grazed down to a 4cm stubble height.

There was a small compensatory growth effect once these animals were rehoused which meant that the animals that grew more slowly during the grazing season, grew faster in the shed. This reduced the 24kg liveweight difference to 14kg at the point of slaughter. This translated to an 11kg heavier carcase weight for the cattle that had grazed to a stubble height of 6cm compared to those that grazed to 4cm.

Therefore, the conclusion of this study was that grazing too tight (4cm) restricted animal growth at pasture.

What is interesting from these two studies is that the recommended pre- and post-grazing sward heights for beef cattle vary slightly from what is recommended on dairy systems. Commenting on this difference, Eddie said research carried out on beef systems over the years would have consistently seen a reduction in animal performance once post-grazing sward height goes below 5cm.

Grassland management

He suggested that as far as grassland management is concerned, grazing swards in April, May and early June on beef farms is quite forgiving in terms of animal performance, even where pre-grazing yields are greater than target.

It is only in the second half of the grazing season that problems can appear in these pastures that have been mismanaged in early season where we are asking animals to graze out swards which will lead to poor late-season performance. It is for this reason that the recommendations may have previously been to continuously hit lower post-grazing sward heights throughout the season.