IT is now widely accepted grazing animal performance and the water soluble carbohydrate (WSC) content of grass are inextricably linked. Research has shown high grass sugar content not only drives intake by increasing palatability and digestibility but also improves the efficiency of dietary nitrogen utilisation.

According to Professor Mike Theodorou, head of plant, animal and microbiological science at the Institute of Grassland and Environmental Research (IGER), the water soluble carbohydrate content of grass is one of the major limiting factors in the efficient use of feed nitrogen by ruminants.

The amount of microbial protein produced for use further down the digestive tract is largely dependent on the amount of energy available to the rumen bacteria. If the amount of energy can be increased by feeding higher WSC grass in the diet - and more use therefore made of forage protein - there is potential to reduce input costs or increase output from existing levels of supplementary feed, he says.

Until recently the most effective way to increase the sugar levels of grazing swards was to include tetraploid ryegrass varieties in the grass seed mixtures as they tend to have a higher WSC than the majority of conventional diploid varieties. In addition tetraploids have an open, upright growth habit, which encourages high levels of intake. Although this open growth habit is a strength it is also a weakness of tetraploid varieties as it makes them less tolerant of poaching damage. As a result, tetraploids are normally grown in combination with diploid perennials whose more compact growth characteristics help to increase the persistency and durability of the sward.

Mixing tetraploid and diploid ryegrass varieties has been seen as a trade off between improved persistency and lower overall sward sugar contents. But as research is now showing, the new high sugar varieties developed by plant breeders at IGER offer livestock producers the best of both of worlds by improving sward durability and improving on the WSC content of tetraploid varieties.

Ongoing research carried out by Dr Trevor Gilliland at the Plant Testing Station at Crossnacreevy in Northern Ireland is showing the best of the new generation of high WSC perennial ryegrasses can more than hold their own against the established tetraploid varieties.

The trials which are now in their third year have monitored the water soluble carbohydrate levels under simulated grazing conditions of a number of widely grown tetraploid and diploid perennial ryegrass varieties which feature prominently on the NIAB Recommended and National lists.

Although the varieties have been comparable on overall yield the trials have highlighted a wide variation in their WSC content.

As expected, the tetraploid varieties have had the edge over the conventional diploid varieties on both sugar content and yield with the best achieving a WSC content of 203g/kg. The top performer in the trial has been IGER's breakthrough high sugar variety AberDart.

In 2003 this diploid perennial not only matched the tetraploids on overall dry matter yield at 15.8t/ha but also topped the league on WSC content at 205g/kg.

"In subsequent trials AberDart has been consistently on a par or better than the tetraploid varieties tested," comments Dr Gilliland.

While there was a relatively stable ranking for sugar concentration throughout the season, AberDart maintained a clear advantage over the other varieties at virtually all seven simulated grazing cuts.