By Phillipa Butler
The incorporation of dual-purpose wheats into grazing systems in higher rainfall areas could lead to a significant increase in productivity and profitability.
Field studies in these areas have shown that liveweight gains by stock grazing winter wheat can often be greater than those achieved on forage oats or on the pasture that the crop replaces.
It also appears that grazing dual-purpose wheats does not reduce grain yields, provided animals are removed before the stem elongation stage. In a number of experiments, grain yields after grazing were actually higher than in ungrazed crops.
CSIRO Plant Industry lead researcher Dr Hugh Dove points out that the "positives" of using dual-purpose wheats have to be balanced against the loss of grazing between the time pasture is sprayed out and the time stock can graze the wheat ("the crop penalty").
This can be a period of eight to 12 months. In higher rainfall areas, some of the grazing lost by pasture removal can be recouped by sowing a brassica forage crop in the spring, between pasture removal and the sowing of the wheat. This may also help with disease control in the wheat.
Some of the findings emerging from Dr Dove"s recent GRDC-funded research relating to the role of brassica forage crops indicate that:
CSIRO"s research addressed two main questions: what drives the animal response to grazing winter wheats, and what are the direct and hidden costs of using winter wheats.
"There is little objective data with which to answer the question of animal response, especially in Australia," Dr Dove says.
Winter wheats are used extensively as cool-season pasture in the US and there, very high liveweight gains have been observed. For example, one study in Arkansas compared the liveweight gains of 230 kilogram steers grazing eight cultivars of winter wheat for 144 days at 2.5 steers/ha.
Across cultivars, daily gains ranged from 1.47kg to 1.76kg/head (529 to 633kg/ha).
"We have little comparable data in the Australian context, where dual-purpose cereals tend to be used as winter forage crops rather than as "the pasture"," says Dr Dove.
A comparison in the NSW Southern Tablelands a few years ago, of lambs grazing two varieties of winter wheats and forage oats, provides some indication of the potential of dual-purpose wheats. On average in that study, animals consuming winter wheat forage grew 22% faster than those consuming forage oats.
Grazing the oat cultivar Blackbutt, the lambs gained 282 grams per day; grazing the winter wheat Tennant they gained 369g/day, and on Gordon winter wheat, 320g/day.
"Rapid liveweight gains such as these can only occur if the nutritive value of winter wheat forage is better, or if animals consume more of the winter wheat, or if the nutrients from winter wheat are utilised better once consumed (or a combination of these)," Dr Dove"s report says. "We have scant data about any of these, and currently cannot assess what causes the observed high rates of gain."
In a second study, the liveweight gains were disappointing - 236g/day. This illustrates that although liveweight gains on winter wheat forage are often very high, it is not always the case, notes Dr Dove.
"Unfortunately, without objective data on the nutritive value and intake of cereal forage or pasture, it is not possible to establish the reason for the variability in observed liveweight gains of animals grazing winter wheat."
Dr Dove found that the major direct cost of grazing a dual-purpose wheat is a possible effect on grain yield. However, "in our studies to date, this does not appear to be a problem. In the first grazing trial, grain yields following grazing were either slightly higher or unaffected."
In a trial at Cowra, NSW, grazing Tennant winter wheat caused a small reduction in yield, from 3.29 to 3.05t/ha, but in recent work near Harden, grazing of EGA Wedgetail wheat resulted in a 20 to 30 percent increase in grain yield.
Liveweight gains of lambs grazing forage oats or two cultivars of winter wheat, in the southern Tablelands of New South Wales*
* Grazed by 39.5kg crossbred lambs from early July to early August. Initial herbage allowance 40kg dry matter/lamb (from Dove et al. 2002)
Provided animals are removed from the crop before stem elongation occurs, there should be little cost in terms of grain yield.
Of greater concern, as a "hidden cost" in the cropping/grazing system, is "the crop penalty" - the negative effect of the pasture grazing lost between removal of pasture by winter cleaning in one year, and the grazing of the winter wheat in the subsequent year.
This may be enough to cancel out any increases in animal performance from crop grazing.
One way to recoup some of this lost grazing might be a spring/summer forage brassica crop in the winter-cleaned area, before sowing the winter wheat in February/March. This could bring two advantages - the provision of high-quality forage for grazing in spring/summer, when pasture quality is declining, and a possible "break-crop" effect on soil-borne wheat disease, especially take-all.
This issue is being examined at Ginninderra Experiment Station, near Canberra. Here, areas destined to be sown to winter wheat or to forage oats were either left fallow over spring/summer, or sown down to Hunter hybrid brassica - a rape-turnip hybrid - in mid-September.
Crossbred lambs of around 36kg began grazing the plots in mid-December, when the average herbage mass was 4.7 tonnes of dry matter (DM) per hectare.
"Liveweight gain from 15 December to 8 January was excellent (282g/day), and by January 8, the brassica crop had provided 1600 grazing days/ha and the average weight of lamb produced was 445kg/ha."
However, this liveweight gain comes at a cost, says Dr Dove, and that is the brassica crop"s use of water from the soil profile which would otherwise have been available to the subsequent wheat or oat crop.
"Assuming the lamb liveweight gain is worth about $1.70/kg liveweight, the value of the gain produced from the brassica is about $750/ha," he says.
"Assuming that every reduction of 1mm in soil water content costs 20kg grain, valued at $160/tonne on-farm, the extra water-use by the brassica crop has cost the system at least $225/ha.
"The difference ($750 to $225) suggests it is profitable to include the brassica crop in the rotation, but the calculations assume that the wheat crop has been successful. If the reduced soil water content due to the brassica crop is enough to cause poor early growth or even complete failure of the wheat crop, then the costs would be much higher and could eliminate any gains from grazing brassica."
For more information:
Dr Hugh Dove, 02 6246 5078, Hugh.Dove@csiro.au
GRDC Research Code: CFP 00009, program 4
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