The following article appeared in the Summer 2019 edition of the FarmLink quarterly member-exclusive publication The Link (archives appear at http://www.farmlink.com.au/the-link).
While it might be tempting to allow weeds and crops that have been cut for hay or windrowed to regrow as a source of livestock feed, the impact on following crop yields could be significant.
Riverina Independent Agronomy consultant Neil Durning says weedy canola crops are ideally sprayed before windrowing to reduce the addition of ryegrass to the seedbank and a loss of moisture and nutrients from the soil.
“As part of an integrated weed management plan, it is worth applying glyphosate to canola with ryegrass from early senescence at label rates,” he says. “One way or another you want the canola and the ryegrass dead. It doesn’t take much rain for ryegrass to stick a head out and add more seeds to the seedbank.”
Mr Durning says wheat following canola where glyphosate was applied before harvest appears to hang on for longer in dry finishes. Spraying before windrowing also reduces the likelihood of plant regrowth after rain.
3D-Ag consultant Peter McInerney advises his clients against grazing sheep on paddocks that have been cut for hay.
“Do not try to retrieve residual grazing from hay paddocks because there is none to be had,” he says. “All you will do is powder the soil and have it blow away, particularly paddocks that have been cut for hay for two consecutive years.
“A crop that would have produced 2.5t/ha of grain leaves 3 to 3.5t/ha of residue. That same paddock cut for hay will leave behind less than 1t/ha of residue.”
Where hay has been cut for two consecutive seasons, Mr McInerney encourages the application of manure to replace lost organic matter and nutrients.
Effective summer weed management is critical to reap the soil water conservation benefits of retained crop residues (Flower, Dang & Ward 2019).
Zeleke (2017) showed that summer weed control increased residual soil water and soil nitrogen by 64 millimetres and 60 kilograms per hectare respectively.
Lilley and Kirkegaard (2007) used modelling to show that summer weed control could increase subsequent wheat yield by up to 20 per cent.
According to NSW Department of Primary Industries researcher Colin McMaster (https://weedsmart.org.au/how-much-moisture-and-nitrogen-is-wasted-on-weeds-over-summer/) trials in Central New South Wales showed the economic benefit of every dollar per hectare spent on herbicides to control summer weeds was $8/ha.
Mr Durning says mixed farmers must treat weeds as if there are no livestock in the system, particularly if the paddock is earmarked for cropping.
“The minute you compromise on summer weed control by allowing weeds to grow large is when you start to reduce the yield potential of next year’s crop,” he says. “Preserving moisture over summer is the difference between having a crop that can be harvested and salvaging a failed crop in a dry finish.”
John Stevenson allows sheep to lightly graze stubbles on ‘Orange Park’ near Lockhart only after the first summer knockdown has been applied. His actions back research showing grazing sheep on crop residues at low stocking rates has no detrimental impact on following crop yield (Hunt et al. 2016, Allan et al. 2016).
Where paddocks are bare, Mr Durning says a strategic cultivation may be needed to curb erosion and maximise water infiltration, although this depends on slope and implement choice. Cultivation leaves the surface coarse and lumpy to slow run-off and reduce surface wind speed.
Mr Condon says one of his clients with discs on 16.5cm row spacings planted millet over summer as a cover crop.
“It was sprayed out at early tillering and we saw no yield loss in the following crop even in a dry season,” he says. “You have to be disciplined to avoid grazing and spray it out before the roots reach 15 to 20cm deep to prevent stored soil water loss.”
While two to three tonnes of wheat stubble per hectare (Kirkegaard & van Rees 2019) or 70 per cent soil cover are suggested to minimise erosion and maximise water infiltration, Mr Condon suggests keeping 100 per cent of cover if possible.
He and Mr McInerney agree that confinement lots are worthwhile to preserve soil cover. Confinement lots work on the theory that less topsoil is lost by concentrating sheep in a small area rather than a large area.
Mr McInerney suggests positioning a new confinement lot near an existing tree line where there is standing crop and to apply for grants for infrastructure needs.
Mr Condon says confinement lots allow sheep to be efficiently kept at target condition scores using grain and straw, while preserving soil cover.
“My clients with diverse rotations who kept stubble from 2016 have produced reasonable crops during the past two years,” he says. “The retained stubble allowed autumn rain to infiltrate rather than run off and enabled crops to be established on time.”
Mr Durning agrees, adding that he has noticed paddocks with less stubble cover failed two weeks earlier this year than paddocks with a higher percentage of cover.
Acknowledgements: Peter McInerney, Neil Durning, Greg Condon and John Stevenson.