Now you've got it, how to kill it? by Brendon Cant

Spring spraying, with a follow-up herbicide or cultivation treatment, may be the lucerne grower's best bet for killing the plant prior to a cropping phase.

Ironically many growers who have mastered lucerne establishment are now finding that removing it is equally challenging.

Problems in doing so, according to project leader Mark Peoples, CSIRO Plant Industry, are widespread in eastern Australia and were identified as an emerging problem for WA growers at last year's Lucerne 2000 Symposium in Katanning. Profitable perennial pasture/cropping systems depend on some answers.

Growing plants easier to kill

Farmer experience and early trial work have indicated that cultivation and continuous grazing alone are relatively ineffective at removing lucerne.

"Generally, herbicides are considered effective at removing up to 80 per cent of lucerne, but most growers consider this level of control unsatisfactory," he said.

A miracle chemical option is unlikely, so the focus has fallen on timing of herbicide applications to maximise kill, regardless of what herbicide mixes are used.

With lucerne's ability to move into and out of transient, drought-induced dormancy, herbicide

absorption, translocation and efficacy can be dramatically affected. Actively growing plants are more likely to absorb and transfer the chemical internally, and are therefore more likely to be killed.

"Spraying in early or mid-spring, when there is adequate water and growth is rapid, seems to increase the likelihood of a good kill," he said.

Similarly, lucerne plants should be allowed to regrow for about three to four weeks after grazing or cutting, before a herbicide is applied. "This is when sugars from photosynthesis are directed towards the roots to replenish the root reserves that were used to support the initial shoot regrowth. This increases the chance of the roots and crown receiving a lethal dose of herbicide," said Dr Peoples.

Save the glyphosate

Of the chemical mixes themselves, auxin hormone-like herbicides such as 2,4-D, MCPA, clopyralid (Lontrel) and Grazon have proved to be successful, if sprayed at the optimum time.

"A lot of farmers have been using glyphosate in that mix as well, but it seems glyphosate may have only a limited effect on lucerne, assisting more in the control of associated grasses," Dr Peoples said. Further research is currently under way to confirm this preliminary observation.

Timing is everything

Results from testing herbicide timing at Borden in WA revealed substantial yield and quality benefits for a following wheat crop when a high-percentage kill is achieved.

A spring spray with a follow-up cultivation reduced the population to less than one plant per square metre and was followed by a wheat crop that yielded 6 t/ha, with 13.2 per cent protein.

In contrast, a December spray followed by cultivation left the population at 10 plants per square metre, with a wheat yield of 4.5 t/ha, and protein of 11.2 per cent.

These results support the eastern Australian trials, which suggest that a combination of herbicide and other measures, such as grazing, hay cutting, or cultivation, should form the basis of the overall lucerne removal strategy.

The project is supported by growers and the Federal Government through the GRDC.

Programs 3.5.2, 3.5.3 Contact: Dr Mark Peoples 02 6246 5244 email