Picky Eaters – Cows Prefer Certain Grass Varieties by Thomas Turbritt

Within my studies, which are part of the AFBI-Teagasc-QUB PhD programme focusing on improving grassland production in Ireland, I recently visited a farm involved in the Teagasc on-farm variety evaluation study.

In this study farmers sow individual varieties of perennial ryegrass in paddocks and record the performance of the paddock. However, due to an expanding herd size, one dairy farmer, who had sown two individual varieties in adjoining paddocks had to remove the fence between the two varieties.  In this circumstance, it was very interesting to see that the cows always grazed one side (one variety) of the field before grazing the other. This is one example of cow’s preference for one grass variety over another.

A further example of preference is when cows dislike a variety and they have no problem letting the farmer know. Cows will bellow and roar to get the farmers attention and will not settle in the paddock.  A knock on effect is that the paddock is poorly grazed with a lot of grass left behind. This poor graze-out increases the proportion of stem in the sward at the next grazing.  This is less digestible than leaf and can depress milk production. Farmers may mechanically top these poorly grazed paddocks to maintain quality but this is both a labour cost and a loss of grass from the diet.

With the high cost of reseeding estimated to be up to €750/ha, it is very disappointing for farmers if a new grass sward is unpalatable to cows. One farmer I met told me of one new reseed on his farm where his cows are never happy and as a result, he never leaves the cows grazing at night for fear that they will break the wire.  Therefore, grass varieties are now being examined in a new trial in Teagasc Moorepark to investigate their graze-out potential.

Thirty perennial ryegrass varieties from the DAFM Recommended List were sown in 36m² plots and rotationally grazed from February to November on 11 occasions. On average the trial grew 15t DM/ha over the year. Before grazing the yield and height were measured and samples collected for digestibility. The ‘after-grazing’ heights were measured with a rising plate meter as the measure of graze out.  So swards with lower post-grazing heights had greater levels of utilisation.

The variety consistently achieving the lowest post-grazing height across the year was Astonenergy at 3.7cm on average and the highest was Clanrye, a full 1cm higher at 4.7cm. The average post-grazing height across the varieties was 4.2 cm.

Varieties with larger pre-grazing heights were found to have poorer graze-out and from this a new characteristic of grazing quality called the Residual Grazed Height (RGH) was calculated.  Again, Astonenergy and Clanrye performed best and worst for RGH respectively. However, Clanrye had the highest and Astonenergy the lowest amount of DM yield over the year.

Assessing variety performance for graze-out and DM yield showed varieties like Aspect, Twymax and Abergain to be the best compromise varieties as they gave high DM yields with a tight graze-out. The graph shows that tetraploid varieties were among those grazed-out the most, with diploids generally yielding well but not grazing-out tightly. The majority of tetraploids had better graze-out performance than diploids even when DM production was similar between both ploidies. Abermagic (Diploid) and Kintyre (Tetraploid) both yielded about 13.8 t DM across the year but Kintyre grazed-out more tightly.

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