A field study was conducted from 1991 through 1993 on a Gigger silt loam soil (fine-silty, mixed, thermic Typic Fragiudalf) to evaluate the effects of tillage systems and winter cover crops on N fertilizer requirements of cotton (Gossypium hirsutum L.). Tillage systems were no-till (NT) and conventional-till (CT). Cover crops were native vegetation, hairy vetch, and winter wheat. N rates were 0, 35, 70, 105, and 140 lb/A. The experimental design was a split plot in a randomized complete block with four replications in 1991 and three replications in 1992 and 1993. Main plots were tillage systems and sub-plots consisted of a factorial arrangement of cover crop and N rate combinations. Plots were 8 rows (40-inch spacing) 45 feet in length. The test was irrigated with a linear move sprinkler system.

Hairy vetch and wheat cover crops were drill seeded into standing cotton stalks between mid-October and early November each year. Seeding rates for vetch and wheat were 25 and 90 lb/A, respectively. Cover crop dry matter yield and N concentration (%) were determined from biomass samples collected from each plot one to two days prior to growth termination with tillage (CT treatments) or herbicides (NT treatments). Dates of termination were April 24, 1991 and April 10 in 1992 and 1993.

Stoneville 453 cotton was planted at six seed/foot of row on June 4, 1991, May 4, 1992, and May 7, 1993. All plots were planted with a John Deere 7100 planter in 1991 and 1992 and with a John Deere 7300 planter in 1993. Ripple coulters were mounted ahead of each planter unit for the no-till treatments.

N fertilizer rates were applied as 32% U.A.N. solution 17 to 21 days after planting each year. The fertilizer was applied as a surface dribble band 10 inches from the drill and incorporated with a cultivator in 1991. In 1992 and 1993 the fertilizer was injected three inches deep and 10 inches from the drill.

The N concentration in vetch residue averaged 4.1, 3.5, and 3.9% in 1991, 1992, and 1993, respectively. Dry matter yield of vetch residue averaged 2304, 2820, and 3186 lb/A in 1991, 1992, and 1993, respectively. Residue N content (N concentration times dry matter yield) averaged 93 lb/A in 1991, 99 lb/A in 1992, and 124 lb/A in 1993. Neither N concentration nor dry matter yield of vetch was influenced by cotton tillage or N rates.

The N concentration of wheat residue averaged 1.2% in 1991, 0.9% in 1992, and 1.0% in 1993. N concentration in wheat tissue was not influenced by tillage or N fertilization rates. In 1991 the average dry matter yield of wheat residue was 3935 lb/A. Wheat dry matter yield was increased with the 105 and 140 lb/A N fertilization rates as compared to 0 N in 1992 and 1993. Wheat dry matter yield at the 105 lb/A N fertilization rate in 1992 and 1993 averaged 4689 and 3145 lb/A, respectively. The total N content of the wheat cover crop in 1991 averaged 47 lb/A. In 1992 and 1993 the wheat cover crop residue at the 105 lb/A cotton fertilization level averaged 42 and 32 lb/A of N, respectively. Cotton tillage systems had no effect on wheat dry matter yield.

Mature plant height of NT cotton was taller than CT in 1991 and 1992 but not in 1993. In 1991, cotton following vetch and wheat was significantly taller than cotton following native vegetation. The 70 and 105 lb/A N rates (averaged across tillage and cover crops) produced significantly taller plants than 0 N in 1991. No interactions between tillage, cover crops, and N rates were observed for plant height in 1991. Significant interactions between cover crops and N rates were observed for plant height in 1992 and 1993. In 1992 and 1993, plant height of cotton following native vegetation was maximized with 70 lb/A of N. Cotton plant height following vetch was not influenced by N rates in 1992. In 1993 cotton plant height following vetch was increased with 35, 70, 105, and 140 lb/A N compared to 0 N. Cotton plant height following wheat was maximized with 70 lb/A of N in 1992 and with 105 lb in 1993.

Tillage systems had no effect on cotton yield in 1991, 1992, or 1993 and the tillage x cover crop and tillage x N rate interactions were not significant. The cover crop x N rate interaction, however, was significant each year. Cotton yield following native vegetation was optimized with 35 lb/A of N in 1991 and by 70 lb/A in 1992 and 1993. Cotton following wheat cover crops required 105 lb/A of N for optimum yield in 1991 and 1993. In 1992 yield was optimized at 70 lb/A; however, yield tended to increase (non-significant) with 105 and 140 lb/A. Cotton yields following hairy vetch were not increased by N fertilization in any year. Cotton following native vegetation, hairy vetch and wheat cover crops produced similar yields at optimum N fertilization levels.