Capeweed control on lucerne at a cost?

Growers Stuart and Brian McAlpine and agronomist Richard Olive watch the growth of lucerne on the McAlpines' 'Cooinda' property.

CONTROLLING capeweed during a pasture phase now looks achievable with the results of trials from the Low Recharge Cropping Systems Project in Western Australia, but not without some cost.

At Stuart McAlpine's property 'Cooinda' , west of Buntine in Western Australia, the trials aimed to assess capeweed control options in lucerne pastures in an area with a rapidly exploding capeweed population.

"Capeweed became the focus of these trials because an unusually wet summer in 1999- 2000 allowed the capeweed to become a significant problem in our lucerne even before the break of season," says Mr McAlpine.

Capeweed is not usually a problem so early in the season. However, after four years of growing lucerne over large areas, Mr McAlpine has seen tremendous variation in the amount and timing of rainfall and the different weeds that it can bring with it.

"Because lucerne is growing whenever moisture is present, we need a range of weed control options over the entire year," says Mr McAlpine.

Supported by AGWEST and growers through the GRDC, the trials tested nine herbicides, applied both before and after the break of season. On top of the early winter cleaning treatments, three spring seed-set control practices (herbicide, mowing and none) were included as part of the trials.

According to AGWEST's Diana Fedorenko, an effective capeweed control option for growers with an unusually early capeweed problem was to apply glyphosate at rates of 0.6 and 1.0 litres per hectare before the break of season.

Both application rates reduced weeds to less than 10 per cent of their pre-spraying plant number, but not without some short -term pain.

The higher application rate of glyphosate severely burnt lucerne tops, reducing the cover (a measure of plant size) to almost half," says Dr Fedorenko. Even so, the plants recovered well, reaching 65 per cent aerial cover by the middle of the spring, and continuing to improve. Just as importantly, these plots were the cleanest of any treatment tested.

Applying even low rates of a glyphosinate-ammonium herbicide (BASTA) after the break was an effective option. "When applied after the break of season, this herbicide killed most capeweed plants, without affecting significantly the number and size of lucerne plants," says Dr Fedorenko.

Dr Fedorenko indicated that spray experiments over two years of lucerne and crop establishment have given different results on different soil types. "Although these differences could be explained in terms of stored soil moisture, root depth and pasture health, we require further data collection and processing to explain these effects."

Related strategies

Mr McAlpine is just one of many growers turning to lucerne as a means to help reduce recharge in landscapes affected by waterlogging or salinity. Although capeweed control quickly became the focus of the trials, the project initially aimed to test the feasibility of an early winter clean on pure stands of lucerne.

"Although I would normally grow pure stands oflucerne, I would now consider spraying early in the season with a glyphosate herbicide and then oversowing with a mixture of Serradella cv. Cadiz and oats," says Mr McAlpine. Mr McAlpine said this approach brings two benefits.

Weed germination is promoted by tickling the soil surface while on a lucerne pasture, which will exhaust seed banks of herbicide-resistant weeds for future seasons. It also produces good feed to offer a more balanced diet to his livestock.

Program 3.4.4 Contact: Dr Diana Fedorenllo 06 9690 2228

A more detailed version of this research was first reported at the 2001 Farming Systems for Sustainability AGWEST GRDC Crop Update.