By Alec Nicol
If you have a four-year-old lucerne stand you are about to spray out before cropping, Ted Lefroy asks why? He wonders what would happen if you simply sowed the crop over the lucerne. "Yes," he concedes, "you"ll suffer a yield penalty, but how much?"
Dr Lefroy argues that our system of phase farming is inefficient on a number of fronts. We have an unpredictable climate so timing the move from pasture to crop is a risk; lucerne is difficult to establish and remove; and the need to manage the water table never stops. So what if we simply over-crop a lucerne paddock?
"Simplistically, if we look at a six-year phase farming rotation with three years under crop and three years under lucerne, we"re at best achieving 50 percent of potential crop yield over that time," he says. "If we were able to continuously crop over lucerne for that period and suffer less than a 50 percent yield penalty we"d be in front."
Dr Lefroy and Dr Yvette Oliver are working at Dumbleyung in the southern wheatbelt in WA. Creeping dryland salinity is a focus, with the debate raging over mechanical intervention in the form of drains or vegetative intervention.
They set out to learn how effective an over-sown lucerne crop would be in managing water table recharge, and the impact it would have on grain yield.
They sowed plots on four different soil types on grower Jeff Patterson"s property. Everything from a heavy clay through to a sandy soil and a duplex soil is represented, and he has over-sown lucerne plots "cheek by jowl" with plots devoted to conventional phase farming.
"It"s important to know just how much lateral influence lucerne has on managing recharge," Dr Lefroy says.
"Yes, there is a yield penalty and as you"d expect it"s greatest in the dry years, but across the three years on all plots it"s less than 40 percent. "However, the real story comes in the impact that lucerne is having on the recharge."
Dr Lefroy has found that it is the age of the stand, rather than plant density that is important. "Plant density decreased over time but the buffer created by the lucerne increased.
"Even a stand with as few as five lucerne plants per square metre would control recharge, and we estimated that you"d need 60 millimetres more rain on an over-sown crop before the subsoil was as wet as under a conventional crop."
Significantly, he determined that the lucerne has a lateral impact on drainage. On heavy clay soils, that influence extended between one and two metres, and on the duplex soils the influence was measurable half a metre away.
"That opens up the possibility of a system of establishing lucerne in rows two to four metres apart in heavy soil and a metre apart in the duplex soils and still adequately managing the recharge."
Jeff Patterson has been sowing a paddock to lucerne each year for a number of years. When he saw the results, he speculated on a system of alternative strips of baler-width lucerne and header-width crop.
However, there are still a number of unanswered questions, says Dr Lefroy:
"There"s very little green matter; about 200 to 300 kilograms per hectare, but nutritionists tell me this would allow livestock to make better use of standing stubble."
Green material at harvest could be a potential nuisance but Jeff Patterson believes the value would outweigh any inconvenience.
"We"ve found that lucerne"s ability to control recharge is much better from the second year of establishment, so perhaps we have to look at crop being over-sown in the second year," he says. "Then we"d need to suffer no more than 40 percent yield loss to be square with phase farming."
For more information:
Dr Ted Lefroy, 08 9333 6442, Ted.Lefroy@csiro.au
GRDC Research Code: UWA 345, program 4