Biomass holds key for stock grazing timing
Biomass assessments should be used with phenology rules of thumb in deciding when to remove stock from crops. This is the message from a three-year GRDC-funded CSIRO project with a focus on optimising the grazing and yield potential of dual-purpose crops.
CSIRO researcher Dr John Kirkegaard says that current rules of thumb are not necessarily optimising the balance between grazing and grain yield.
“Based on the phenology rules you would lock up wheat before GS30 to avoid removing developing heads, or canola before the buds elongate 10 centimetres from the ground. This is sound advice, but can be refined,” Dr Kirkegaard says.
“We have found that, with care, it’s possible to graze past these points without yield penalty. But importantly, it is also possible to lose yield if the grazing has been too heavy, even in these ‘safe’ windows when high grain yield is expected. The key is understanding how residual biomass is related to yield expectations.”
Achieving a given yield target requires a certain level of biomass at flowering. For a wheat yield of five tonnes per hectare, about 9t/ha of biomass will be required at anthesis. If grazing is managed to leave sufficient biomass at lock-up to achieve 9t/ha, then grazing will not limit yield.
Field experiments conducted as part of the project involved grazing wheat and canola, with stock removal at three dates: mid-July, late-July and early August. As well as grazing up to each date, portions of the crop were mowed to provide 0.5, 1.0 or 1.5t/ha of biomass. Varietal differences were also tested, including triazine-tolerant canola compared with hybrid or conventional canola and different wheat varieties.
“We found, somewhat surprisingly, that the different varieties did not vary much in the recovery from grazing when the phenology stage at lock-up was carefully matched. So all three types of canola recovered to the same yield potential after grazing was halted. The hybrid canola did provide increased feed for the stock before that stage, so was still a better overall outcome,” Dr Kirkegaard says.
In most situations, he advises that well-managed stock grazing will not reduce grain yields, with the exception being in very high-yield-potential situations.
“In most of south-eastern Australia, where wheat yield averages are around 3 to 4t/ha, there is often excess biomass, so by locking up the crop at an appropriate stage, growers can get the benefit of grazing without a yield penalty,” he says.
“In dry areas, where the yield potential may be only 1 to 2t/ha, grazing can actually reduce crop water consumption over winter and give a slight yield benefit. However, where potential yields are very high, the plant will need a lot of biomass to achieve that high yield, so there may be a reduction from grazing.”
The required timing and biomass for optimised stock removal depends on each grower’s circumstances. The project is developing a simple calculator where growers can input their target yield potential and location, and the tool will specify what residual biomass is required at a certain date, helping growers to decide when to remove stock.
“The intent is that our tool will have photos so growers can assess the level of biomass visually compared with their paddocks and choose when to take off stock. They can also run a scenario where they can see the likely result of leaving stock on for, say, another week, as the tool will compare any impact on yield with the gains from grazing,” Dr Kirkegaard says.
The grazing tool is in a testing phase and should be available for use in the 2016 season. In the meantime, Dr Kirkegaard suggests some general rules growers can apply (see Figure 1 for representations of the biomass levels):
- always avoid removing reproductive parts during grazing (heads for wheat, elongated buds for canola);
- in 3 to 4t/ha yield situations for wheat, growers can leave 0.5t/ha of biomass for lock-up times up to the end of July;
- if the stock is left on until mid-August, more biomass will be needed as the crop is more advanced, up to about 1.7t/ha by mid-August (however, do not remove elongating heads);
- spring canola needs to be locked up by the end of July, with about 1.5t/ha residual biomass supporting 2.5 to 3t yields;
- earlier-sown winter canola will naturally produce much higher levels of biomass and should be locked up at the end of July with 2.5t/ha biomass; and
- more or less biomass will be required depending on the yield potential of the crop and the lock-up date.
Dr John Kirkegaard,
0448 354 630,
End of Ground Cover issue #117 southern edition
Read the accompanying Ground Cover Supplement:
GRDC Project Code CSP00160