A model for pest assessment in New Zealand sheep pastures

Abstract The simplest possible simulation model of a sheep flock breeding system is used to translate the effects of pasture pests into production and revenue losses. Using the New Zealand Hepialid moth, porina, as an example, the percentage reduction in pasture growth over an eight-month period is equal to half the porina density (m −2 ). Monetary losses are about 10 c per stock unit for each larva m −2 or a reduction in stocking rate of 0·4% per larva m −2 if attacks are sustained. Thresholds for control with insecticide, the cheapest method of treatment, are 40 m −2 and 20 m −2 for stocking rates of 10 and 20 SU ha −1 and double these values for sporadic attacks on under-stocked farms. This assumes 100% return or NZ$2 per NZ$1 spent, rather than the 0% implied by the economic threshold. A distinction is drawn between ‘grazing’ and ‘denuding’ pests, having absolute and proportional effects respectively on pasture growth, and between long-term or chronic problems which affect carrying capacity and short-term acute problems which depress individual animal production. Percentage monetary losses from long-term pest problems equate with measured percentage pasture growth losses. Expected losses from short-term, sporadic attacks are similar but more sensitive to within-farm stocking rate. Maximum possible losses in years of poor pasture growth, occurring with a probability of 25%, are in the order of 25% greater than the expected losses. The proportional effect on animal production and revenue of any seasonal pasture loss (or gain) is best estimated by averaging the seasonal percentage losses or by weighting the percentage loss by the proportion of the year over which it occurs, rather than by expressing the absolute loss as a percentage of annual production.

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