The relationship between the grazing efficiency and the production, morphology and nutritional traits of perennial ryegrass varieties

Abstract

Ruminant grazing systems aim to optimize the proportion of grazed forage within the diet and perennial ryegrass grazing efficiency influences both the quantity and quality of herbage utilized. The objective of this study was to examine morphological and chemical plant characteristics and test for any associations with grazing efficiency. The leading perennial ryegrass varieties from the 2016 Irish Recommended List were established in a plot study and rotationally grazed by dairy cows over 3 years. Pre-grazing plant characteristics were measured and related to grazing efficiency, as measured by ‘residual grazed height’. Data were analysed using the PROC MIXED procedure to test for trait differences between varieties, and their corresponding ploidy and heading categorizations. Traits displaying significant variety variability were then tested for their correlation to grazing efficiency using the PROC GLM procedure. Tetraploid varieties exhibited superior grazing efficiency over diploids, due to their superior performance in traits correlated with grazing efficiency. Increasing organic matter digestibility and leaf proportion and decreasing neutral detergent fibre and stem height within the sward were found to increase grazing efficiency. The observed link between these plant traits and grazing efficiency indicates a possibility that they could be used to develop proxy measures to aid breeders to select for, and evaluators to measure, varietal grazing efficiency without involving animal assessments. It was concluded that this would make it logistically practical to assess large numbers of varieties in small plot trials to both breed and evaluate perennial ryegrass varieties for superior grazing efficiency.

Agronomic performance of ten perennial ryegrass varieties on commercial grassland farms

“…(2017) found on-farm grazing DM production to be a reliable indicator of the DM production advantage of individual grass varieties due to a strong correlation between total and grazing DM production of varieties. In recent analyses of PRG grazing traits, it was documented that tetraploid varieties were more suitable to grazing than diploids as they tend to have advantageous nutritive and morphological characteristics, including higher values for dry matter digestibility and increased levels of leaf in the sward, which lead to greater herbage utilisation (Byrne et al ., 2018; Tubritt et al ., 2020 a ). Variety grazing event differences do not appear to be explained by ploidy differences in the current work.…”

Incorporation of the grazing utilization subindex and new updates to the Pasture Profit Index

“…This acceptance is evident in the increased sales of high-ranking varieties compared with lower-ranking varieties within the PPI list (unpublished data, David Cummins, Department of Agriculture, Food and the Marine, Ireland). The key Tubritt et al (2020b).…”

“…Their larger cell size and greater ratio of cell contents to cell wall proportion (Burns et al, 2012) lends itself to higher levels of digestibility compared with diploids (Wims (Byrne et al, 2018). Despite this correlation, digestibility only explained 50% of the variation in RGH between varieties in Tubritt et al (2020b) and therefore, although digestibility is already captured in the PPI quality subindex, the additional variation justifies the inclusion of the utilization subindex. The low rank correlation (0.54) between the original PPI quality ranking and utilization ranking of varieties further demonstrates that both traits are independent measures.…”

“…The selection criteria used by perennial ryegrass variety breeders is influenced by the evaluation protocols used to develop variety recommended lists (Stewart and Hayes, 2011). The inclusion of the utilization subindex will incentivize breeders to increase selection on traits with known correlations to increased grazing efficiency, such as increased OM digestibility, increased leaf proportion and reduced sheath height (Byrne et al, 2018;Tubritt et al, 2020b).…”

“…Pre-and postgrazing sward heights were measured at each grazing rotation with a Jenquip rising plate meter (Jenquip Rising Plate Pasture Meters; diameter 355 mm and 3.2 kg/m 2 ). The agronomic results from this study are described in Tubritt et al (2020b).…”