Predetermining factors for grazing efficiency differences in perennial ryegrass varieties

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Predetermining factors for grazing efficiency differences in perennial ryegrass varieties

T. Tubritt, N. Byrne, T.J. Gilliland, Luc Delaby, M O’Donovan

To cite this version:

T. Tubritt, N. Byrne, T.J. Gilliland, Luc Delaby, M O’Donovan. Predetermining factors for grazing

efficiency differences in perennial ryegrass varieties. Joint 20. Symposium of the European Grassland

Federation and 33. Meeting of the Eucarpia section ”Fodder Crops and Amenity Grasses”, Jun 2019,

Zürich, Switzerland. �hal-02736085�

524 Grassland Science in Europe, Vol. 24 – Improving sown grasslands through breeding and management

Predetermining factors for grazing efficiency differences in perennial ryegrass varieties

Tubritt T.

, Byrne N.

, Gilliland T.

, Delaby L.

and O’Donovan M.

1

Animal and Grassland Innovation Centre, Teagasc Moorepark, Fermoy, Co. Cork, Ireland;

Institute of Global Food Security, Queen’s University Belfast, Belfast, Northern Ireland;

Agri-food and Biosciences Institute, Hillborough, BT26 6DR, Northern Ireland;

INRA, AgroCampus Ouest, UMR Physiologie, Environnement et Génétique pour l’Animal et les Systèmes d’Elevage, 35590 Saint Gilles, France

Abstract

Perennial ryegrass (Lolium perenne L.; PRG) is the principal forage species sown in Ireland for ruminant production. This forage is predominantly fed as grazed grass and has been shown to be the most efficient feed for Irish ruminant production systems. Anecdotal evidence exists from farmers claiming that grazing livestock show preference for certain perennial ryegrass varieties. The aim of this study was to determine what varietal traits in perennial ryegrass influence grazing efficiency by examining differences in grazing intensity between varieties. Grazing efficiency was defined as the proportion of leaf tissue grazed, relative to that presented. Fifty-five PRG varieties were assessed. The plots were rotationally grazed by dairy cows for four grazing seasons from 2015 to 2018 inclusive. Varieties were shown to differ in their level of grazing utilisation (P<0.001). The average difference across the four years between the most and least grazed variety was 0.85 cm (3.72 and 4.54 cm respectively). Tetraploid varieties as a group were shown to have a greater utilisation performance than the diploid group (P<0.001) with mean Residual Grazed Surface Heights (ResGSH) of -0.15 and +0.11 respectively. Increased digestibility and a more open sward structure were associated with varieties found to have greater graze-out potential.

Keywords: cultivars, grazing, post-grazing, sward height, utilisation

Introduction

A key improvement to be made within Irish ruminant production systems is to employ PRG varieties that perform superiorly within grazing systems, as Irish farms depend hugely on grazed pasture (O’Donovan et al., 2018). Increased feed intakes, improved pasture quality and even, tightly grazed swards are advantages of superior grazing varieties. Low post-grazing sward height (≤4 cm) is indicative of good grazing performance as this increases pasture intake and conditions the sward to increase leaf production which has a higher feeding value relative to stem and dead proportion (Stakelum and Dillon, 2007). PRG variety evaluation is conducted by the Irish Department of Agriculture, Food and the Marine (DAFM), which publishes a Recommended List. Plots are evaluated by mechanical defoliation on a rotational basis to mimic Irish rotational grazing systems. Mechanical harvesting protocols may be limited in the information they provide as varieties are employed on farm in a different system than they have been evaluated in. Creighton et al., (2012) found no significant difference in herbage yield between varieties of PRG managed under cutting or animal grazing protocols. However, Cashman et al., (2012) found a similar relationship between cut and animal grazed protocols for herbage yield but significant differences (P<0.001) in dry matter off-takes were observed between varieties under animal grazing. The aim of this study was to identify PRG varieties that perform superiorly in grazing systems. PostGSH was used as the measure of grazing efficiency and varietal traits likely to be responsible for differences in grazing efficiency were also investigated.

Materials and methods

In August 2014, 55 varieties of PRG of varying heading date and ploidy were sown in Teagasc Moorepark,

Co. Cork in a randomised complete block design in 3 replicates. The varieties were both recommended

Grassland Science in Europe, Vol. 24 – Improving sown grasslands through breeding and management 525 and candidate varieties from the Irish DAFM Recommended List trials. The 7×3 m (21 m

) plots were grazed once during their establishment phase in 2014 and data were collected from 2015 onwards. The plots were grazed on 32 occasions from 2015 to 2018. The 1-hectare paddock was rotationally grazed by lactating dairy cows when herbage mass was estimated to be 1,300 kg DM ha

. Prior to grazing the pre-grazing surface height (PreGSH) of each plot was measured using a Jenquip rising plate meter. The plate meter also measured the PostGSH once the cows left the paddock and this was used as the measure of grazing efficiency. Herbage samples were collected prior to grazing events 2-7 in 2015 and 3-7 in 2016 to calculate sward nutritive quality (Dry matter digestibility (DMD), crude protein (CP) and water- soluble carbohydrates (WSC)). These were harvested with Gardena hand shears at 4 cm above the base of the sward, dried at 60 °C for 48 hours, milled and scanned by near infra-red spectrometry (NIRS).

Morphological traits were examined in 2015 and 2016, once in the vegetative stage and once in the reproductive stage; 20 tillers were used to determine extended tiller height (ETH) and extended sheath height (ESH). Free leaf lamina (FLL) was calculated by subtracting the ESH from the ETH. Each tiller was then dissected into its leaf, stem and dead (LSD) proportions and oven dried at 90 °C for 24 hours to express each fraction on a dry matter (DM) basis. Data were analysed in the statistical program SAS 9.3 (SAS Inst. 2011). PROC MIXED was used to analyse the effects of years and grazing events as fixed terms on PostGSH, DMD, CP, WSC, FLL and LSD proportions. Block was included as a random variable in the model. The relationship between PostGSH and the variables analysed in the previous mixed model were examined using PROC GLM. Residual grazed surface height (ResGSH) was calculated for each variety and averaged over ploidy group (Tubritt et al., 2018).

Results and discussion

Pre and Post GSH differed significantly between the varieties. The mean PostGSH across all the varieties was 4.19 cm. Pearson rank correlation calculated between all years for PostGSH was 0.62. PostGSH differed significantly between ploidy. Tetraploids, on average as a group, had significantly lower PostGSH than diploids at 3.99 and 4.32 cm respectively. Figure 1 displays the difference in PostGSH between tetraploid and diploid varieties.

PreGSH was significantly and positively correlated with PostGSH (R

=0.48) (Table 1). Diploids had higher PreGSH and therefore higher PostGSH compared to tetraploids. ResGSH accounts for the PreGSH effect on PostGSH, allowing for accurate comparison between varieties. Negative ResGSH values indicate that varieties have good grazing efficiency. Overall, the tetraploid group had superior grazing efficiency to diploids with in average a ResGSH values of -0.15 and +0.11 respectively, though the very best three diploids were grazed similarly or more tightly than the weakest tetraploids. Tetraploids were found to have significantly higher levels of DMD, CP, WSC and FLL than diploids (P<0.01). High values in these traits were allied with low ResGSH (P<0.01).

Figure 1. Difference in post-grazing sward height of diploids and tetraploids.

526 Grassland Science in Europe, Vol. 24 – Improving sown grasslands through breeding and management

Conclusion

The assessment of PRG varieties under animal grazing showed that varieties differed significantly in the level to which they are grazed by cows. Previous research has shown that grazing to low PostGSH increases the leaf proportion and nutritive value of the sward (McCarthy et al., 2013). This study also provides evidence that tetraploid varieties were more conducive to grazing systems as they achieve lower ResGSH values. Genetic diversity existed within ploidy groups showing that further progress in grazing efficiency is possible. Assessment of PRG varieties using animals is costly and therefore identification of easily measured PRG traits for grazing efficiency is worthwhile. FLL, DMD and CP were three traits identified in this study that plant breeders could use to select for varieties with greater grazing efficiency.

The study also suggests that farmers choosing grass varieties for grazing systems should select as high a proportion of tetraploid varieties within their seed mixtures as conditions and management permit.

References

Cashman P.A., McEvoy M., Gilliland T.J. and O’Donovan M. (2016) A comparison between cutting and animal grazing for dry matter yield, quality and tiller density of perennial ryegrass cultivars. Grass and Forage Science 71, 112-122.

Creighton P., Gilliland T., Delaby L., Kennedy E., Boland T. and O’Donovan M. (2012) Effect of Lolium perenne sward density on productivity under simulated and actual cattle grazing. Grass and Forage Science 67, 526-534.

McCarthy B., Pierce K.M., Delaby L., Brennan A., Fleming C. and Horan B. (2013) The effect of stocking rate and calving date on grass production, utilization and nutritive value of the sward during the grazing season. Grass and Forage Science 68, 364-377.

O’Donovan M., Hennessy D. and Creighton P. (2018) Ruminant grassland production systems in Ireland. Sustainable meat and milk production from grasslands. Grassland Science in Europe 23, 17-25.

Stakelum G. and Dillon P. (2007) The effect of grazing pressure on rotationally grazed pastures in spring/early summer on the performance of dairy cows in the summer/autumn period. Irish Journal of Agricultural and Food Research 46, 29-46.

Tubritt, T., Gilliland, T., Delaby, L., McHugh, N. and O’Donovan, M. (2018) Comparison of grass utilisation performance of perennial ryegrass varieties. Sustainable meat and milk production from grasslands. Grassland Science in Europe 23, 54-59.

Table 1. Trait differences between diploid and tetraploid groups under animal grazing for key selected traits.

Trait Diploid Tetraploid Significance

PreGSH (cm) 10.00 9.60 ***

PostGSH (cm) 4.32 3.99 ***

ResGSH +0.11 -0.15 ***

Free leaf lamina (cm) 19.0 20.5 ***

Dry matter digestibility (g kg

DM) 819.8 833.2 ***

Crude protein (g kg

DM) 176.4 178.8 **

1 P>0.05; * P<0.05; ** P<0.01 *** P<0.001.