CLIMATIC RISK FOR SWEET CORN PRODUCTION IN CANTERBURY, NEW-ZEALAND

The risk of sweet com (Zea mays L.) crops failing to reach harvest mat urity as a result of climatic variability among seasons was quantified for four locations or the east coast of New Zealand's South Island. R esults of field experiments in the region showed that three representa tive cultivars had thermal time requirements from planting to maturity ranging from 1215 to 1320-degrees-C days above a base temperature of 6-degrees-C. In simulations, crop success or failure was estimated for combinations of five hypothetical cultivar maturities and four planti ng dates spanning the ranges likely to used in the region. Calculation s used temperature records that ranged from 17 to 25 seasons depending on location. A crop was judged to fail if either its thermal time req uirement was not met by 30 April or it was frosted before the requirem ent was met. Allowance was made for the fact that some ''failed'' crop s were sufficiently close to maturity to be harvested successfully. Ri sk was negligible at Blenhiem, the most northerly location, where seas onal mean temperature between 15 October and 30 April was 15.8-degrees -C and the first autumn frost occurred late. Timaru, the most southern location, had earlier frosts and a 13.7-degrees-C man temperature, an d risk of failure was high except for early plantings of early-maturin g cultivars. Lincoln, representing the intermediate area of central Ca nterbury, had a mean temperature of 14.6-degrees-C and a high risk of failure for late plantings of late-maturing cultivars. This location i s probably at the climatic limit for commercial sweet corn production with an acceptable level of risk, using currently available cultivars. A warming trend since 1928 has resulted in a substantial decrease in risk over the period. The analysis highlighted the marginal nature of the Canterbury climate, the effect on risk of the sensitivity of the c rop's development rate to temperature, and indicated the potential ben efit of any further climatic warming.