The heat unit concept is being used as a method of monitoring the rate of growth and development of the cotton plant. Based on a minimum developmental threshold of 60 F, DD-60's are calculated daily. The accumulative values are made available to growers for use as a guide to compare their crop's development at various growth stages with that of models prepared from data generated during the past several years.

Data from an 8-year study at Stoneville showed the following average DD-60 accumulations during the various developmental stages, from planting: emergence, 62; first square, 478; first bloom, 874; peak bloom, 1623; first open boll, 1898; and 60% open bolls, 2479. The data from this study also indicated that the distribution of heat units during the growing season had a greater influence on yield than did total heat unit accumulation. A negative linear relationship was found between total DD-60 accumulation and lint yield, but positive correlations existed between yield and DD-60 accumulation at 10, 20, 30, and 40 days after planting; the correlation between yield and DD-60 at 20 days was highly significant. These data were collected from early-maturing cultivars at a single location; other data, collected from various locations throughout the Mid-South, show a positive linear relationship between total heat unit accumulation and lint yield. Both yield and DD-60 accumulation increase as the collection points are moved from north to south in the Mississippi Delta. The contrast between correlations of these one-location and multiple-location studies may reflect differences between micro-environmental and climatic effects on cotton physiology.