A drip irrigation system was used to apply water daily to treatments in multiples of evapotranspiration (ET) from 1985 to 1988 at the Delta Branch Experiment Station, Stoneville, Mississippi. ET was calculated from pan evaporation data along with pan and crop coefficients. Water was applied beginning at the first sign of visual stress after first bloom and ending between first open boll and two weeks after first open boll. During the irrigation periods, irrigation and rainfall totaled 42, 53, 63, 74, and 92% of ET in 1985; 34, 47, 65, 75, and 97% in 1986; 50, 68, 87, 105, and 123% in 1987; and 64, 78, 89, 110, and 118% in 1988 for the five irrigated treatments. The variety, IDES 4221, was solid planted and grown on a Bosket very fine sandy loam soil using recommended cultural practices.

All irrigated treatments in all years produced significantly higher lint yields than the nonirrigated treatment. In general, lint yields increased as the total water applied with irrigation and rainfall approached the estimated value of ET for the period of irrigation in the years of 1985, 1986, and 1987. The treatments receiving 80 to 100% of ET (92, 97, and 87%) for 1985, 1986, and 1987 produced lint yields of 1321, 1046, and 1367 lb/A, respectively. These yields were significantly greater than yields from treatments receiving less than or equal to 53, 75, and 68% of ET in 1985, 1986, and 1987, respectively. In 1985, the 92% ET treatment yielded significantly more than the 74% treatment, but its yield was not different from the 63% treatment. In 1987, the 87% ET treatment yield was numerically higher than the 105 and 123% ET treatment yields but was not significantly different. The 3-year average estimate of ET (1985 to 1987) was 0.24 inch/day during irrigation periods that lasted 36, 24, and 38 days, respectively.

The irrigation period in 1988 lasted 56 days, which was at least 18 days longer than irrigation periods of the previous years. The five irrigated treatments in 1988 produced significantly higher lint yields (average = 1068 lb/A) than the nonirrigated treatment (837 lb/A) but there were no significant differences in yield among the irrigated treatments. This suggests that we over-irrigated in 1988.

In conclusion, lint yields increased as the total water applied with irrigation and rainfall approached the estimated value of ET when the duration of the irrigated period was 38 days or less.