Deferred grazing as a low-cost method of pasture conservation and renovation is nothing new, but a farmer-led Sustainable Farming Fund project is quantifying its benefits.

Deferred grazing, or summer fallow, takes ryegrass-based pasture paddocks out of the grazing rotation over late spring and summer to allow them reseed and build root reserves before being grazed in autumn.

Typically carried out on hill country where mechanical harvesting of surpluses is impossible, taking a paddock out of rotation allows grazing to be concentrated and therefore pasture quality retained on the balance of the farm.

Trial work carried out on this pasture management tool over 20 years ago suggests giving pastures time to recuperate results in longer residuals, deeper roots and a lot more clover, all of which have benefits in summer dry or volatile climates.

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Through the Sustainable Farming Fund project, Farming in a Volatile World, two Bay of Plenty hill country sheep and beef farmers - Allen Coster from the Kaimais and Brian Thomas from Te Puke - are working alongside AgResearch, Ballance Agri-Nutrients, Plant and Food Research, Bay of Plenty and Waikato Regional Councils and Beef + Lamb New Zealand to quantify the impact deferred grazing has on pasture cover and quality, soil properties, root mass and the farm finances.

Last year a trial plot was set up on the Coster's farm in the lower Kaimais comparing the effect of standard rotation grazing with two deferred grazing treatments. One was closed in November and opened in January after one missed grazing while the other was closed in November and not opened until March, after ryegrass had set seed.

There's also a glasshouse study to help understand the mechanisms that lead to ryegrass growth and survival under deferred grazing.

Farmax modelling is also being carried out to understand the effect deferred grazing has on pasture quality and livestock production. Initial results showed the late opening improved some soil parameters such as bulk density and total porosity, ryegrass tiller densities and the proportion of ground cover and reduced facial eczema (FE) spore count. While there was an expected reduction in the nutritive value of these saved pastures by May, after two grazings by dairy heifers after the end of the deferred period, the nutritive value was similar across all treatments.

This was indicative of the high proportion of fresh green grass.

The timing of grazings appeared to be a critical factor to maximise seedling survival and establishment.

If pastures are opened up for grazing too early seed-set of desirable species may not have occurred and if pastures are opened up and grazed too late, seedlings may be trampled and damaged by livestock.

Both scenarios would reduce the establishment of seedlings from the seedbank.

It was a particularly wet late summer, although January was very dry, on Allen's farm this year and the trial will continue at the same site and the hypothesis tested under another season where climatic conditions will possibly be different.

The science team is also planning to set up a trial in a summer dry farm environment.

For Allen, summer fallow is a cost-effective method of maintaining pasture quality of the rest of the farm. It also rejuvenates the pasture under fallow and Allen says these treatments grew an extra 90kg DM/ha in the two months after they were opened. In late winter these treatments grew an extra 280kg DM/ha which is worth $54/ha.

"It's quite significant compared to other methods of managing summer surpluses."

He used dairy heifers to graze the treatment blocks immediately after they had been opened up and says they gained 0.5kg/day cleaning up the surpluses.

Allen says to do this, farmers really need a class of cattle where there are no imperatives to maximise growth rates at that time of the year - such as breeding cows or cull dairy cows.

Other summer fallow treatments had some pugging damage from the previous winter and the long vegetation suppressed Californian thistles that would normally appear in damaged soils.

Prior to opening the treatment up, a second crop of thistles did appear but Allen says they were obviously very palatable as the heifers grazed them to the ground.

AgResearch scientist Katherine Tozer says they did not find any differences in the cover of thistles between the deferred and other treatments in the replicated plots and this difference in reducing thistle population in different situations is something that they may look at this summer.

- Rick Burke is also chairman of Farmers For Positive Change.