The effect of altering autumn grazing management on sward structural characteristics, morphology and nutritive value

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Looney, Caitlin
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University College Cork
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Altering grazing management practices to increase the amount of herbage available on farm can help extend the grazing season, improve herbage utilization and, in turn, improve the sustainability and efficiency of pasture based dairy systems. Extended grazing happens at two pivotal time points - autumn and spring. Management for both incorporates building herbage masses; in autumn, the grazing rotation is extended from August to a peak in October, which in turn increases pre-grazing herbage mass. Increasing herbage in spring relies on accumulating herbage, over winter achieved by autumn closing date (CD). Accumulating herbage mass has the potential to impact various factors of sward production, morphology and physiology. The current thesis evaluated how management practices in autumn and spring affect the perennial ryegrass (Lolium perenne, L; PRG) plant. This research is important to identify areas where management can be improved which will allow for the increase in herbage available to be utilised, improve efficiencies, profitability and sustainability. The objective of this thesis was to investigate the impact of accumulating herbage masses for the extension of the grazing season in autumn, over winter and spring. Herbage masses in autumn were investigated at different defoliation dates (DD) across the final rotation (chapter 2 and 3). The effect of final defoliation in autumn (CD) over winter and into spring was investigated (chapter 4 and 5). It was found accumulating herbage should begin in August and herbage masses > 2000 kg DM ha-1 should be defoliated before 7th November, as there is a large decline in herbage mass and sward quality. Low target herbage mass (THM; 500 kg DM ha-1) should have a later DD, as swards continue to accumulate herbage, while maintaining sward quality, however, low THM, can potentially be carried over winter, average farm cover is below farm targets for spring demand (i.e. above 750 kg DM ha-1; S.R. > 2.9 livestock units (LU) ha-1). Carrying swards with lower herbage masses over winter was demonstrated successfully in chapter 4 and 5. The medium THM, can maintain sward quality across the autumn period, despite losing herbage, so can be grazed later in the final rotation. Sward morphology was also affected by increasing herbage masses in autumn. Light transmitted to the base of the sward was reduced, which resulted in increased internode elongation. Very high THM had the greatest number of tillers with internode elongation and the longest length of internode elongation. Autumn CD was found to affect herbage production, herbage quality, leaf, stem and dead proportions and tiller density over winter and in spring. Autumn CD is responsible for 50% of the herbage available in spring, with each day delay in closing resulting in an extra 16 kg DM-1. Swards closed earlier had a higher herbage mass in spring and lower sward quality compared to later closed swards; that had reduced herbage and a higher herbage quality. Autumn CD effected subsequent growth rates but not quality in spring (2nd rotation); swards closed early had a lower growth rate compared to swards closed later following the initial spring defoliation. Sward structure was negatively affected by early autumn CD. Early CD had greater parent and daughter tiller mortality than later CD. Early CD also resulted in greater leaf extension and leaf senescence rates in daughter tillers compared to late. The results of this thesis have provided further insight into the grazing management practices in autumn and spring and the impact they have on sward structure, quality and herbage. These results will help improve grazing management practices into the future.
Perennial ryegrass , Autumn grazing management , Sward structure , Herbage production
Looney, C. 2021. The effect of altering autumn grazing management on sward structural characteristics, morphology and nutritive value. PhD Thesis, University College Cork.