Massey University professor of dairy production systems Danny Donaghy said that ryegrass was essentially a group of tillers. They’re the functional units of the grass plant and have their own leaves and roots although they’re connected at the base to other tillers.
The tillers have a lifespan of about a year so it’s important for pasture persistence and overall pasture production that grazing and managing the plant promotes more tiller growth.
The parent tiller produces daughter tillers from the base.
Each tiller only has three live leaves and only one leaf grows at a time. That means by the time a fourth leaf emerges the first leaf will die. That cycle continues so that the oldest leaf dies as the new leaf emerges.
The speed at which leaves appear depends on temperature and moisture – there’s no effect from fertility or cultivar but the rate of leaf appearance in annuals is slightly faster than perennials, Donaghy told the winners of Agriseeds Grass into Gold competition.
This three-leaf principal of growth is the foundation of good grazing management.
Photosynthesis in the leaves produces glucose and then water-soluble carbohydrates (WSC) in the plant. They provide energy for the plant to continue growing.
WSC in the leaf can vary significantly with time of day because of the time the plant has had to photosynthesise.
Levels start the day at about 5% and usually peak about 4-5pm at about 14% WSC/kg drymatter (DM).
In Southland, where day length is longer in summer, WSC levels can peak higher.
Any WSC that’s not used right away is stored in the lower part of the tillers, in the base or bottom 4cm of the plant.
Before the start of the fourth leaf stage, both metabolisable energy (ME) and digestibility remain about the same but after the third leaf stage they begin to decline and fibre levels increase as dead leaf and stem material build up.
Mineral balances also change.
Potassium, which is usually at levels beyond the cow’s needs in the first leaf stage, declines as the second, third, and fourth leaves emerge while calcium increases.
A ratio of more than 1.6:1 for calcium and phosphorous is recommended and in the first leaf stage it’s about 1:1. By the third leaf stage it’s more than 2:1.
Basing the grazing rotation or time between grazing each paddock on leaf re-growth stage rather than just pasture height or platemeter readings therefore makes sense for the plant and the animal, Donaghy said.
That’s why grazing at the 2.5 to three leaf stage, just before the third leaf emerging on the tiller, is the ideal.
The time between each leaf appearing varies, depending on temperature and moisture, so in warm late spring conditions it might only be five to seven days but in winter and early spring it might be 15-20 days.
It will vary from week to week and from season to season.
Rule of thumb is at higher growth rates of more than 60kg DM/ha/day graze closer to two leaves and at other times graze closer to three.
Canopy closure is another marker favoured by Donaghy.
It’s when you can’t see the ground or pasture base directly below if you’re looking down onto the pasture from above.
It’s usually when pasture cover is more than 3000kg DM/ha, so graze at the two to three leaf stage unless canopy closure has happened before that.
‘Consistently 40-60% of total high quality feed is grown during the third-leaf stage. Growth rates can be higher after this but quality declines.’
Once the canopy has closed pasture quality will also start to decline with light restrictions, causing yellowing and dying of the lower leaves. Tillering will start to decline and fibre will increase.
How hard to graze is a question that’s also answered by the physiology of the plant and its growth habit. The main growing point for the plant is in the base. Grazing below 4cm, down to 2cm, risks damaging the growing point but the plant will cope with one or two grazings down to that level.
Repeated low grazings will permanently damage it and the plant isn’t likely to persist.
Lax grazing, leaving residuals of 8-10cm, also creates problems. Again one or two lax grazings might not permanently alter the physiology of the plant but after two to three grazing rotations at that level the growing point moves up the plant. New tillers emerge higher up, at the raised growing point, and the base becomes more fibrous.
Timing grazing decisions to coincide with the point at which the plant was in the best physiological state, both in terms of providing maximum quality feed and in terms of positioning itself for best re-growth, was the aim, Donaghy said.
“Good pasture management often requires a leap of faith but you can be secure in the fact that if you manage your grass well, then regardless of climatic factors, you will optimise its potential,” he said.
