Differential dry matter production, nutritive value and grazing utilisation of perennial ryegrass varieties and mixtures evaluated within intensive grazing regimes

  • Tomas Tubritt

Student thesis: Doctoral ThesisDoctor of Philosophy


Grass based ruminant production systems aim to utilise large proportions of grazed herbage as high utilisation levels reduce costs associated with ruminant production and therefore maximise profit. Perennial ryegrass (Lolium perenne L.) grazing efficiency has been shown to influence farm herbage utilisation.

The objective of this thesis was to develop a methodology for the evaluation of perennial ryegrass variety grazing efficiency. Such evaluation data could then be incorporated into the Irish Pasture Profit Index (PPI), an economic variety ranking selection index, to incentivise breeders to develop superior grazing varieties. An investigation of easily measured plant traits that could be used by breeders as proxies for increased grazing efficiency was also examined. Finally a comparison between the agronomic performance of perennial ryegrass varieties sown as monocultures and mixtures was conducted to evaluate the use of mixtures.
Residual Grazed Height (RGH) was developed as a measure of grazing efficiency. Significant differences in grazing efficiency were recorded between varieties evaluated under animal grazing, indicating that not all varieties are suited to intensive grazing systems. Tetraploid varieties displayed a clear advantage over diploids for grazing efficiency which was allied with the increased nutritive value (digestibility) and a favourable sward structure of tetraploids. The economic value for increased grazing efficiency was determined based on the additional herbage dry matter that a variety with superior grazing efficiency could utilise. This resulted in a €44 difference between the most and least grazing efficient varieties. The performance of perennial ryegrass mixtures rarely exceeded that of the highest component monoculture but likewise never was inferior to that of lowest.

Routine variety evaluations must evolve to new farming practices and regulations. This thesis outlines a methodology for the assessment of grazing efficiency and its inclusion within the PPI which will allow farmers to increase levels of pasture utilisation on farm.
Date of AwardJul 2021
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SupervisorMichael C. O’Donovan (Supervisor) & Trevor Gilliland (Supervisor)


  • Perennial ryegrass
  • grazing efficiency
  • variety evaluation
  • pasture profit index
  • seed mixtures

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