Featured Journal Paper

From Grass and Forage Science

Autumn 2020

An investigation into the grazing efficiency of perennial ryegrass varieties

Authors: T. Tubritt, L. Delaby, T. Gilliland and M. O’Donovan

Online (DOI): 10.1111/gfs.12481

Grass and Forage Science 75 (3) pp. 253 – 365

Perennial ryegrasses (PRG) are widely used on dairy farms, but differences in grazing efficiency between varieties could affect grassland utilisation and impact upon profitability.  In most EU countries the grazing information included in Recommended Lists of forage varieties is based on a regular cutting regime rather than removal by a grazing animal.  This work examined the differences in grazing efficiency between 30 PRG varieties when grazed rotationally by spring calving dairy cows.  A Residual Grazed Height was calculated for each variety, taking into account a range of factors and including the difference between the predicted post-grazing sward height (PostGSH) and the actual PostGSH.  Those varieties with a negative value for the RGH (where the actual PostGSH value was lower than the predicted value) were described as having good grazing efficiency whilst those with a positive RGH had a poor grazing efficiency. 

Results for herbage production for different varieties confirmed their seasonal categorisation in the Recommended List.  Diploid varieties had higher annual production than tetraploid varieties (12.62 v 11.90 t DM/ha, p < 0.002).  Diploids also had a higher pre-grazing height (10.16 v 9.77 cm, p < 0.001) and higher post-grazing height (4.23 v 3.90 cm, p < 0.001) than tetraploids.  It was emphasised that farmers need to make sure that they do not overgraze swards as this can lead to lower regrowth. 

RGH values ranged from -0.38 to +0.34 across the 30 PRG varieties.  Tetraploid varieties had a better average RGH value (-0.13) than diploids (+0.13).  The authors suggested that this may be confusing i.e. a negative value being better than a positive value, and that converting the value to a ‘positive’ would be beneficial.  Future work will look to incorporate grazing efficiency into the Pasture Profit Index (PPI).

The authors concluded that there are opportunities to breed plants for improved grazing efficiencies due to the differences in grazing efficiency between varieties within each ploidy and suggested that grazing efficiency measures should be included in future selection indices.

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