Crop topping lupins for ryegrass control by Cathy Nicoll

Difficulty for growers with crop-topping is timing: to prevent ryegrass seed-set and also avoid crop yield losses.

WA grower Jamie Greaves has been crop-topping since 1996 to control ryegrass and brome grass. Trials on his property, 'Maisy Downs', have been answering a few basic questions about the likely yield losses when spraying at different stages of crop maturity.

The work is that of the Mingenew-Irwin Group Inc., a large independent grower group in the Geraldton area of Western Australia.

"Crop-topping is a simple matter of spraying a crop with a knockdown herbicide at the critical time—when the crop is mature but before the grass has fully matured," says scientist Cameron Weeks. "The difficulty for growers is spraying at exactly the right time to prevent ryegrass seedset and avoid crop yield losses."

The trials by the Mingenew-Irwin Group measured the yields of two varieties of lupins, Tanjil and Belara, after being sprayed with Gramoxone at a rate of 1 L/ha at two different times (11 October and 20 October). The final yields at harvest are shown below for each of the crop-topping times, and these are compared to yields in control plots that were not crop-topped.

Mr Weeks said the possibility that Belara, an early-maturing lupin variety, is well suited to crop-topping came as good news to growers looking for methods of ryegrass control in the face of herbicide resistance.

Effects of two crop-toppings on yields of Belara and Tanjil lupins. The control was not sprayed.
Control yield — crops not crop-topped (t/ha)2.282.78
Yield for lupins crop-topped on 11 October (t/ha)2.312.28
Yield for lupins crop-topped on 20 October (t/ha)2.642.91

Note: Yields on lupins crop-topped on 20 October were higher than the control due to a slightly better soil as the three treatments were sprayed in large blocks going slightly up a hill.

Timing is everything

The trial results show the importance of timing. The later-maturing Tanjil suffered a yield loss of about 0.5 t/ha, as a result of early crop-topping. Being at a stage of only 5-10 per cent of leaf drop, the crop's seedset was affected by spraying at the earlier date. In other trials, crop-topping the lupin variety Gungurru at 70-80 per cent lupin leaf drop stage resulted in a yield loss of 0.35 t/ha.

"Compare this to the early-maturing Belara, for which yield was not affected by either crop-topping. The variety had already lost almost all its leaf by the time of earliest crop-topping in these trials. By the second spray most Belara seed was yellow at the centre," says Mr Weeks.


Although any yield loss is not ideal, grower Jamie Greaves suggests that a yield penalty in a high-yielding variety like Tanjil might be worth the price of good ryegrass control, as only the earlier spraying would have sterilised all the ryegrass seed. Tanjil also has more resistance to Anthracnose than Belara, a factor that is important to lupin growers.

"There is no point in spraying late and leaving some viable ryegrass seed for next year," says Mr Greaves. "Once the grass is browning off it is a bit late for herbicides to be effective without knocking too much off the crop yield. But it would be worth considering if you had a chronic weed problem."

Mr Greaves uses crop-topping to prevent rye and brome grass seedset for the next season's wheat crop. He also manages these weeds with a range of knockdown chemicals and selective herbicides at the pre-sowing stages, and post-emergent stage in lupins if necessary


Crop-topping doesn't come cheaply. In the case of the Gungurru lupin trials, it totalled around $82/ha after allowing for yield losses, physical damage to the crop from using the Rogator for spraying, and the direct costs of herbicides and labour to do the crop-topping. Even with no yield penalty due to the herbicide, crop-topping costs are still as high as $38/ha.

"With lupins getting $125/t on-farm, and yields generally between 2-3 t/ha, it can add up to quite a bit," says Mr Greaves.

But if the timing is right, the verdict from Mr Greaves and other growers in the Mingenew-Irwin Group is that crop-topping lupins is a worthwhile means of weed control in the right season.

Contact: Mr Cameron Weeks 08 9964 2974, Mr Jamie Greaves 08 9929 1046