Sward utilisation Pasture Profit Index update

The Pasture Profit Index (PPI) is used by the grassland industry to identify and select the best varieties when reseeding. Tomás Tubritt, Laurence Shalloo, Noirín McHugh & Michael O’Donovan, Teagasc Moorepark discuss variety grazing efficiency, plot studies & practical implications of using the PPI

Summary

• The Pasture Profit Index (PPI) is used by the grassland industry to identify and select the best varieties when reseeding
• Grazing Utilisation is a new trait included in the 2021 PPI with variety performance expressed using the ‘Star rating system’.

Introduction

Grazing to post-grazing heights of approximately 4 cm is optimum to maintain/increase sward quality as the grazing season progresses. High grass quality maximises grass utilisation which has a positive impact on profitability and sustainability. Farmers involved in the ‘Teagasc on-farm variety evaluation’ study reported that some perennial ryegrass varieties were easier to graze to lower post-grazing heights compared to others but no indication of a varieties grazing efficiency was available within the Pasture Profit Index (PPI). Plot studies at Moorepark found differences in variety grazing efficiency, and these results were used to generate a new grazing utilisation trait within the PPI.  

Variety plot grazing evaluation

Plots of different grass varieties were grazed by dairy cows when average pre-grazing herbage mass was 1400 kg DM/ha. At each grazing event pre-grazing height was recorded on each plot using a rising plate meter. Dairy cows then grazed all plots at the same time to an average post-grazing height of 4 cm. Cows had free access to select what varieties they wished to graze. Once grazed, individual plot post-grazing height was recorded.

Despite having the same regrowth interval, differences in pre-grazing  yield/sward height was recorded between varieties, due to varietal growth potential and previous post-grazing height. Comparing grazing efficiency based on post-grazing height alone was biased towards varieties with lower pre-grazing height (i.e. low pre-grazing yield = low post-grazing height). Residual Grazed Height (RGH) measures the grazing efficiency of varieties, taking into account pre-grazing yield/height differences between varieties. Figure 1 displays the RGH of the varieties currently on the Irish Recommended List. Negative values of RGH are desirable as this indicates that a variety was grazed lower than expected, thereby showing high grazing efficiency. Tetraploid varieties dominate the left (negative value) side of Figure 1 indicating that they are grazed more efficiently than diploid varieties. 

Grazing utilisation sub-index and using the PPI

The RGH data of varieties was used to develop the Grazing Utilisation sub-index within the 2021 PPI (Appendix 1). A variety’s grazing efficiency is expressed in the Grazing Utilisation sub-index as a star rating. Varieties with five stars are highly suited to intensive grazing, while one star varieties are poorly grazed by cows. When choosing varieties for paddocks intended for intensive grazing (e.g. those located on the milking platform), varieties performing strongly in grazing utilisation, quality and spring/autumn DM sub-indices should be selected. Variety selection for paddocks destined for regular intensive silage harvesting would benefit from prioritising silage and spring yield traits. Research investigating variety mixtures found that the trait performance of a mixture could be accurately predicted as the average of the included varieties for all traits.

Practical implications

Poor graze-outs can be a major source of dissatisfaction for farmers, particularly where paddocks have been recently reseeded. The Grazing Utilisation sub-index can guide farmers to select varieties with high grazing efficiency. Tetraploid varieties are more utilisable than diploids which is linked to their higher digestibility and more open growth habit which creates a more appealing sward for cows to graze. High proportions of tetraploid varieties should be included in seed mixtures destined for intensive grazing. This is also true for wetter soils, as the additional benefits that tetraploids swards bring to dairy production systems (higher quality and grazing efficiency) outweigh any perceived deficiencies in persistency. On-farm evaluation of varieties conducted across a number of farms in Ireland have shown little difference in variety persistency over time.

Tomás Tubritt, Teagasc Technologist provides more information in the video clip below.

If you liked this article you might like to read more about the Pasture Profit Index here

This article was produced as part of the two-day Virtual Dairy Conference 2021 which took place on 23 and 24 November.

The complete Dairy Conference Proceedings can be viewed here and the individual articles will be published on Teagasc Daily throughout the week.