Understanding factors associated with the grazing efficiency of perennial ryegrass varieties

Herbage utilisation is of primary importance to grassland farmers due to its strong association with profit, yet it is not assessed in routine grass evaluations. On commercial grassland farms, grazing efficiency of varieties is believed to influence their level of herbage utilisation. To understand grazing efficiency the interaction between plant and animal is critical; mechanically simulated grazing protocols are limited in the information they provide. The objective of this paper was to further understand the grazing efficiency of perennial ryegrass varieties by identifying grazing traits which can be used as selection criteria by breeders striving to develop varieties with improved herbage utilisation. Fifty-five perennial ryegrass varieties, both diploid and tetraploid with intermediate and late heading dates, were sown in plots (3 m x 7 m; 21 m(2)) following a randomised block design with three replicates per variety. Swards were rotationally grazed by lactating dairy cows during the 2015 and 2016 grazing seasons. Sward structural, morphological, and chemical characteristics were measured and related to grazing efficiency, as measured by post-grazing sward height. These data were analysed using the PROC MIXED procedure to test the fixed effects of traits and the GLM procedure to test their relationships with grazing efficiency. Varieties were shown to differ in their level of grazing efficiency (p < 0.001). Tetraploid varieties had significantly (p < 0.001) greater grazing efficiency than diploids recording post-grazing sward heights of 3.8 and 4.1 cm, respectively. Increased free leaf lamina (p < 0.001), tiller mass (p < 0.05) and dry matter digestibility (p < 0.01) were shown to significantly improve grazing efficiency of varieties. This study has identified free leaf lamina, tiller mass and dry matter digestibility as key selection criteria for plant breeders to base their developments upon if grazing efficiency is imported into their breeding programmes.