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Management for persistence of perennial ryegrass in marginal environments

The previous management for perennial ryegrass to improve persistence was to graze at the 2.5 to 3 leaf growth stage. The variability in the weather that we have experienced in the last decade has necessitated the development of a new grazing management system.

In the first year of establishment of a perennial ryegrass farmers are encouraged by agronomists not to cut for hay. This is due to the plants’ roots still developing. The ratio of shoots to roots in ryegrass is usually 2:1. So the larger you can let the shoots grow before you cut or graze them, the deeper the roots will grow. Hay is usually cut late in the season when moisture is running out. The effect this has on the plant is that it will try and achieve the 2:1 shoot to root ratio. This means when moisture is limited after being cut for hay there will be little shoot regrowth and the roots will shrink back to be closer to the soil surface. This can be a problem when the upper soil horizon dries out and can lead to plant death when the moisture runs out.

By allowing the plant to continue growing from spring into summer when moisture becomes limited it will have a deeper root system to allow better survival through a dry summer. We also know that the tillers that survive through the summer on perennial ryegrass are called the secondary tillers.

These are small tillers that develop from the primary tillers that are the reproductive tillers on the plant. Once the primary reproductive tiller has produced seed and dies the secondary tiller carries on through the summer.

Under heavy grazing over the reproductive stage of the plant there are very few secondary tillers produced on the plant that help with its survival over the summer. The time of heading is also important.

If you have an early heading variety it is able to produce primary and secondary tillers before moisture runs out at the end of the season. If you have a late heading variety and the moisture runs out early then it is unlikely that the variety will be able to produce secondary tillers to help it survive through the summer. In most environments in Australia where perennial ryegrass is mostly grown an early flowering variety that would reach the secondary tillering stage would be anything before +8 heading date (relative to Nui). Anything after +8 would be at risk of not producing secondary tillers in an early spring finish.

The later the heading date of the variety the less likely they are to produce viable seeds at heading to allow for seedling recruitment in the following year. Heavy grazing in late spring and summer can set up an autumn of the following year where annual grass weeds  establish quickly in the open drill rows between plants.

A new management strategy may be to let perennial ryegrass plants go to head in their first year and produce viable seed. This  encourages deeper root growth in the first season. The plants can then be grazed through summer as dry standing feed up to autumn to allow seedling recruitment. The high number of perennial grass seeds produced will out compete the other annual grass weeds that are present.

We know that grass seed can remain viable for up to two years in the soil so If you were to repeat the perennial ryegrass plants setting seed every 3 years this could help with a greater persistence of a long term pasture.

Evidence of this method working can be seen through the 2007 drought around Hamilton, Victoria. This location is on the peripheries of where perennial ryegrass can survive.

Image: 2018 Dryland trial plots Valley Seeds Research Station, Yambuck, showing poor survival of Perennial Ryegrass. Photo taken May 2018.

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