Cover crops – Part 2 of 3: Eliminate weeds and reduce fertiliser use
Improving and maintaining soil health is not an overnight fix, but a lifelong endeavour that requires patience and experimentation. In this second part of a three-part series on cover crops and soil health, US farmer and cover crops coach Steve Groff shar
Aspirant and existing conservation agriculture farmers need to appreciate that improving and maintaining good soil health is a never-ending process. During this lifelong journey, there will be times when they experience hurdles and failures in their conservation agriculture actions. Instead of throwing in the towel, they should understand that every setback is temporary and, with research and persistence, can be overcome. So says US farmer and cover crops coach Steve Groff.
“I’ve been practising conservation agriculture for 35 years. It’s a long time. As long as I’m alive, I expect to be constantly learning in my journey of achieving and maintaining good soil health on my farm.
“Nowadays, even if my farm has heavy rain, there’s very little run-off, no erosion, and high moisture infiltration. This is due to soil cover and the constant presence of living plants on my fields. But when I first started with no-till and cover crops, it took me a number of years to achieve these benefits.”
Groff urges farmers to go out to their lands when it rains to personally observe whether the soil is capturing the rainwater rather than allowing it to run off, taking topsoil, nutrients and moisture with it.
A proliferation of weeds resistant to herbicides, particularly glyphosate-based products, has been observed in many countries, including South Africa. This worrying phenomenon poses a threat to food security.
• Improving and maintaining good soil health is a lifelong learning process of both failures and successes. • S horter-season cash crop genetics can be used to extend the cover crop production season and biomass. • Once the cover crop is dead, its residue on the soil surface will continue to suppress weed growth. Groff gives the assurance, however, that, with good management, cover crops can help rid croplands of herbicideresistant weeds.
“Cover crops don’t necessarily eliminate the need for herbicides to control weeds, although this is possible. Cover crops and crop residues on the soil surface can, however, significantly suppress weed growth. The constant, longterm use of cover crops and crop residues leads to everdiminishing numbers of weeds, including herbicideresistant types, that are able to germinate and go to seed. This means that, eventually, any remaining herbicide-resistant weed genetics continue to be suppressed in a land.”
In the US, stooling rye has proved highly effective at weed suppression.
Some cover crops will need to be sprayed with glyphosate to ensure they do not compete with the cash crop. But Groff stresses that herbicides should be used strategically like this only where necessary; the cover crop, either living or dead, should always be used as the primary weed suppressant.
One potential option to minimise herbicide use is to plant a frost-susceptible cover crop as soon as the summer cash crop is harvested and before winter sets in. The cover crop should then grow sufficiently before being killed off naturally by frost.
During this growth period, it will have shaded out weeds. Once the cover crop is dead, its residue on the soil surface will continue to suppress further weed growth.
The cover crop should be planted as soon as possible after the cash crop has been harvested. In all likelihood, residues of fertiliser and soil moisture from the cash crop in the soil will enhance cover crop germination and growth. These nutrients will be recycled into the soil again once the cover crop is dead.
The development of shorter season/faster maturing maize and soya bean genetics offers an opportunity for conservation agriculture farmers to generate an income while maximising the benefits of inter-season cover crops.
Groff points out that shorterseason genetics may not be suitable for all crop farmers but it is worth planting 5% to 10% of total cash crop production to these genetics as a yield trial.
Farmers who conduct their own on-farm research should replicate the trial at least three times on a land, and for at least three consecutive seasons, then harvest the trial crops at optimal maturity to provide accurate yield data. Harvesting a cash crop two to three weeks earlier than normal will, in turn, allow the farmer to plant a cover crop two to three weeks earlier.
“By the time the next cash crop planting season comes around, the cover crop will be two to three weeks older than in previous seasons. This means its biomass, as well as its benefits for soil health, subsequent cash crops and farm profitability, will all be significantly greater,” says Groff.
In his own on-farm trials of shorter-season maize and soya bean genetics, he found that their yields compared favourably with and sometimes even outperformed longer-season genetics. He adds, however, that due to differences in soil types and climate, this might not be the case for all farmers.
Cover crops slow run- off and help to increase water infiltration
the value of legumes
Groff points out that leguminous cash crops and cover crops capture and use remaining soil moisture, as well as capture nitrogen from the air and fix it in the soil for use. They also scavenge any nitrogen remaining in the soil from previous crops. This can be as much as 30kg/ ha, so the farmer does not need to apply as much costly inorganic nitrogen fertiliser to subsequent cash crops.
“Using a precision no-till planter to plant cover crops directly into cash-crop residues straight behind the combine harvester also saves on seed costs. With my precision planter, and with certain cover crop varieties, I can get good germination and equivalent biomass with as little as half the seed rate I would need if I were using a nonprecision no-till planter. This is a big cost saving,” Groff says.
He suggests that South African grain farmers investigate the financial feasibility of harvesting their grain at 18% to 20% moisture content, then having the grain machine-dried to optimal storage and milling moisture content instead of letting the grain dry on standing crops.
Harvesting grain crops two to three weeks earlier and machine-drying the grain will allow these farmers to plant their cover crops two to three weeks earlier, helping the cover crops to maximise their growth and biomass, and hence soil health benefits.
Another method of using cover crops is biostrip-tilling, where two varieties of cover crops, such as a legume and a high-biomass variety, are planted in alternating but closely spaced (say, 28cm) rows in the same land at the same time. Each will provide different benefits for soil health and subsequent cash crops. A subsequent cash crop could then be planted directly into the row where the legume is growing to benefit from the nitrogen that the legume fixed into the soil. The residues of the high biomass cover crop variety will shade out weeds in the cash crop inter-row while also providing beneficial organic matter to the soil.
With careful management, biostrip-tilling also enables a cover crop to be planted in the inter-rows of a cash crop. This helps shade out weeds and improve soil health.
less fertiliser needed
Between his wheat and maize cash crops, Groff grows a diverse multi-species cover crop that contains a significant proportion of nitrogen-fixing legumes. This particular cover crop primarily comprises radish, hairy vetch, sunflower, common vetch, sweet blue lupin, sunn hemp, fenugreek, Austrian winter pea, calendula, oats, phacelia, and fava bean.
“Once this multi-species cover crop is killed off by our cold winters, the earthworms in the field go crazy on the residues,” he explains. “In fact, the first time I tried this, the earthworms ate so much that there was very little residue left. Now
I plant more sorghum in the mix to keep the soil surface covered until planting of the following cash crop.” Almost the entire soil surface can become covered with earthworm castings, which are an organic fertiliser rich in nitrogen.
“My subsequent maize crops achieve profitable yields without my having to add inorganic nitrogen fertiliser.”
a customised mix
Groff cautions, however, that this particular multi-species cover crop is used on lands that are already healthy after many years of conservation agriculture practices. He does not advise novice conservation agriculture farmers to use his cover crop mix, and urges more experienced cover crop farmers to develop a pre-maize multi-species cover crop mix that works for their particular production environment.
• The next and final article in this series will focus on introducing grazing livestock to a conservation agriculture system, and the benefits this offers to soil health. • Email Steve Groff at [email protected]ercropcoaching.com, or visit covercropcoaching.com. • This presentation was given at the 2018 No-Till Conference held from 4 to 6 September in KwaZuluNatal. Visit notillclub.com.
ABOVE: Leguminous crops, such as these beans, can reduce the requirement for inorganic nitrogen fertiliser.