Influence of winter treading on lucerne growth and survival

Lucerne (Medicago sativa L.) is frequently used as a winter run-off in New Zealand. Winter grazing and the associated treading was recommended to reduce weed infestations and reduce over-wintering blue-green aphid (Acyrthosiphon kondoi Shinji) populations. The effect on plant numbers and crown disease was unknown. An experiment was conducted on a free-draining alluvial soil at DSIR, Lincoln, Canterbury, which compared the effects of 1, 2, and 4 weeks' heavy stocking with sheep at a stocking rate of 500/ha and an unstocked control. Treatments were applied for five winters. Treading removed wintering aphids and had no effect on plant numbers, crown vigour, or yield of lucerne at the second spring cut. Severe treading in some years reduced Sitona weevil (Sitona discoideus) larvae numbers and weed growth, but had small effects on lucerne yield at the first spring cut and on crown health. It is concluded that the less severe treading used by farmers can continue to be recommended as the beneficial effects are likely to be considerably greater than the detrimental effects.