Summer cover crops to improve productivity and reduce soil greenhouse gas emissions in State-wide WANTFA trial
The project headed by WANTFA Research Manager Dr Matthew McNee will demonstrate the use of short duration summer cover crops (including tropical legumes and C4 grasses) to reduce nitrous oxide emissions in Western Australian wheat production systems.
Most of the atmospheric losses of soil nitrogen in WA occur after spring and summer rainfall. The predicted increase in ‘out of season’ rainfall raises the prospect of growing summer cover crops to utilise soil N that may otherwise be lost as a greenhouse gas in summer.
WANTFA trialled some hardy crop species in 2012 and 2013 and found that these might be included in cropping systems in a number of innovative ways. For example, summer crops might be sown into residual soil moisture after hay crops, winter fallow or a winter cover crop. Summer crops might also be under-sown with late sown grain or hay crops or managed for self-seeding (i.e. grain is left in the paddock).
Farmers are concerned that summer cover crops will use soil water and nutrients to the detriment of following winter grown crops. However Matthew’s research in central NSW and other recent research in the WA central Wheatbelt suggests that the risks might be managed in some seasons by terminating the growth of the cover crops early after 8-10 weeks growth. Alternatively, a decision could be made to harvest the crops in a wet summer.
Millet and cowpea species were chosen because of their drought tolerance and potential for rapid growth and to increase groundcover. Millet is a fast growing C4 grass and would be expected to have better water use efficiency than cowpeas, a C3 legume. However, legume cowpeas may be of additional benefit when used as a nitrogen-fixing cover crop which then has important implications for water use efficiency.
There will be several study sites across WA. The main trial at Ray Fulwood’s farm near Meckering in the central Wheatbelt will be a fully replicated experiment looking at the comparisons between and management options for White French millet, cow pea crops and fallow. Comparisons will also be made between the impact of summer weeds, the summer crops and a clean fallow. Soil moisture will be measured to a depth of 1.5 metres.
Other potential benefits of the trial may include improvements to non-wetting soil at the Meckering and Bolgart sites.
‘There will be more recent roots in the soil, for water pathways, and potentially more groundcover,’ Matthew said.
Summer crops may be useful to reduce the impacts of water-logging or deep drainage at the trial site at East Kendenup in the south of the state.
Comparisons of interest in the study:
a) Short and long periods of cover crop growth;
b) Grass vs. legume cover crop species;
c) Summer chemical fallow vs. uncontrolled fallow; and
d) Winter cover crop vs. Winter grain crop.
‘A lot of farmers have already been doing it for several years, like Nick Kelly at Newdegate who has been growing millet which now volunteers every year, so we will use strips of his paddock to trial fallow versus summer crops,’ Matthew said.
A key point of interest in this trial is the wheat crop that follows the summer crop. Does this summer crop reduce the accumulation of water relative to summer fallow?
‘Summer crops are going to use some of those resources so we want to know what impact that is going to have on the following wheat crop,’ Matthew said.
Economics will come into play.
‘If you’re losing yield as a result of the summer cropping activity then that is a problem but we might find a significant difference between the costs of spraying your summer weeds compared to summer cropping and fallow.
‘And if we find that we can actually reduce emissions then there will probably be a monetary benefit for keeping the nitrogen in the soil as well.
‘We don’t expect that we can grow summer crops every year, so it’s going to be an opportunistic thing.’
A summer crop can also potentially contribute to lessening a weed problem. If you have a summer weed problem, you can try planting a summer crop and prevent these weeds from growing unchecked. The summer crop is not usually planted with the intention to harvest, but there is the potential to do so.
‘A lot of the time that’s dry sown in anticipation of getting a summer storm so it wouldn’t be a big area of your farm—because it’s a big risk, it would be targeted. The main issue is whether you can get the early growth out of the summer crops in ten weeks. It might not rain in that time and you won’t get growth; in that case you might just keep delaying it. If you do get a wet summer, millet can be $600-800/t so it could balance out the yield penalty costs while adding the soil benefits.’
This project is called ‘Managing fallow cover crops to improve soil fertility and reduce greenhouse gas emissions’. It has been funded through the 2013/14 Federal Government’s ‘Action on the Ground’ program, administered by the Department of Agriculture, Fisheries and Forestry.