Frequent cutting false economy

Press, 1 December 1978, Page 10

Frequent cutting false economy

Cutting lucerne more often than usual to control aphids, instead of spraying, can be false economy, a scientist of the Department of Scientific and Industrial Research has found. More frequent cutting could cost more than $lOO a hectare a season in lost production, according to tests done by Dr M. W. Dunbier, of the Crop Research Division at Lincoln. He has been investigating a suggestion that cutting or grazing lucerne before aphids build up to damaging numbers would be a cheaper form of control than spraying, though it is known that lucerne produces more and lasts longer when allowed to recover after defoliation. The trial was started at Lincoln last season on a healthy stand of irrigated Saranac to compare cutting every four weeks with the normal cutting schedule of every six weeks. Half the trial was sprayed to control aphids and the remainder left unsprayed.

The first cut was taken on October 21, 1977, and harvesting continued until lucerne growth stopped in

May this year. The plots cut four-weekly gave 3.1 tonnes a hectare less dry matter than those cut every six weeks, even when regularly sprayed to control aphids. There was little response to spraying in the trial as there were few lucerne aphids at Lincoln during the season, though previous trials had shown marked yield increases from spraying with insecticides. Dr Dunbier’s results showed the losses from cutting four-weekly for dehydrated meal production in Canterbury that season would have been $124 a hectare, and for haymaking the loss would have been $lO3 a hectare.

Unfortunately, a further adverse effect of cutting four-weekly was found. There were 24 per cent fewer plants at the end of the season compared with the plots cut every six weeks, and the plants remaining were smaller and weighed only two-thirds as much as those from the plots cut less frequently. Even with good management that would mean less production in the following season from the plots cut four-weeklv.

Dr Dunbier concluded that it was false economy to try to save $lO to $2O a hectare a year on sprays by cutting oftener for aphid control when this would cost $lOO a hectare in lost production in a season, plus a weakening of the stand which meant that there might be further production losses in subsequent seasons.