Interest in increased nitrogen (N) rates in combination with plant growth regulators continues in the Mississippi Delta and has been proposed to further enhance cotton productivity for several years. Earlier studies in the Delta showed little response to additional N applied over the rates recommended for particular soils. In many situations, the additional N did not increase lint yield even though some increase in seedcotton yield was observed. The results from the use of plant growth regulators has been varied from year-to-year with no consistent yield response. Quality determination made from samples taken at harvest in the previous study at the Delta Research and Extension Center indicated that high N rates and plant growth regulators reduced micronaire and had adverse effects on color.

A new study was initiated in 1992 and funded in part by the Mississippi Cotton Incorporated State Support Program. The study had a 4 x 4 factorial arrangement of N rates at 90, 120, 150, or 180 lb N/A and mepiquat chloride (Pix) at rates of 0, 2 oz/A applied 4 times, 4 oz/A applied 4 times, and a variable rate and timing to be determined for specific years. The 16 treatments were randomized in a complete block design and replicated four times. The plots were large enough to harvest 20 to 30 lbs of seedcotton from each plot and ginned by the USDA Ginning Laboratory using two lint cleaners. The lint samples were then submitted for quality determination. These determinations include both HVI measurements as well as arealometer and stelometer data. All yield, yield components, and the quality determinations were analyzed statistically using Statistical Analysis Systems (SAS) with Flsd and Waller-Duncan K-ratio t-tests to determine significance.

The first two years of the study have been completed with lint yields in 1992 ranging from 1097 to 1202 lb/A. There was no significant interaction between N rates and Pix rates so main effect means were calculated. Seedcotton yield did not increase above 120 lb N/A when averaged across the Pix rates at the first harvest. There was a decrease in lint percent with increasing N rate. This indicated the importance of examining lint yields instead of seedcotton yield when considering fertility practices. When adjusted for lint percent, there was no difference in lint yield among the N rates. Second harvest lint yields were higher for the 150 and 180 lb N/A treatments both being similar. Total seedcotton yields increased significantly with each N increase up to 150 lb N/A, but when adjusted for lint percent, there was no difference in lint yield above 120 lb N/A. When averaged over N rates, lint yield ranged from 1137 lb/A where no Pix was used to 1168 lb/A where a total of 16 oz/A applied in 4 applications was used . The use of Pix did result in some slight increase in earliness as determined by calculating percent first harvest (PFH = first harvest/total harvest). Fiber properties affected by increasing N rates included decreased micronaire at both first and second harvest, increased Hunter b (more yellowness) for the first harvest and decreasing wall thickness at the second harvest. Fiber length and strength were slightly increased with the addition of Pix and Rd (reflectance) was higher at the first harvest. Micronaire was reduced with Pix but only for the second harvest. Fiber maturity and wall thickness were also reduced by Pix for the second harvest.

In 1993, lint yields were lower and ranged from 815 to 963 lb/A. Similar to 1992, seedcotton yields were higher up to 150 lb N/A but lint percent was lower with increasing N rates. There was no increase in lint yield above 120 lb N/A. Seedcotton yields were higher with Pix but no significant differences were detected for lint yields. This resulted from lower lint percents where Pix was used. The quality components were similar in patterns to those observed in 1992. Micronaire decreased as N rates increased with the range from 4.31 to 4.12 at the first harvest. Fiber length and was slightly higher at both harvests for treatments receiving Pix. Fiber strength in 1993, was not affected by either N rate or Pix rate. Like 1992, fiber maturity decreased with increasing N rates.

After two years, the current research indicated that increasing N rates coupled with plant growth regulator applications has not produced significantly higher yields. Increases in seedcotton yield coupled with subsequent decreases in lint percent did not result in a significant lint yield increase. Pix plant growth regulator produced an earlier maturing crop but increasing N rates delayed maturity. Reductions in some fiber quality components without benefit of increased lint yield suggest that use of higher N rates with mepiquat chloride may not be a profitable practice for all production systems and may penalize producers under the new marketing system.