This is a continuing report on the effects of over -irrigation in cotton production. Started in the Spring of 1995, this study uses drainage lysimeters to study the impact of over-irrigation on nitrate leaching losses. Furthermore, yield and other growth components are monitored to see what effect, if any, the over-irrigation has. The study was initiated at the Maricopa Agricultural Center, Maricopa, Arizona. The drainage lysimeters used are large, open- topped steel boxes filled with soil and placed underground in the experimental field. Crops are grown directly above the lysimeters and the water that moves through the soil profile is collected at the bottom of the lysimeter and analyzed. In this study, two lysimeters were installed. The lysimeters were 80" wide (two row widths), five feet long, and six feet deep. They were placed 18 inches below the soil surface and filled with soil as to best represent the soil in its natural condition. The data presented in this paper are from three years of an ongoing experiment. Throughout the growing season, water samples were taken from the lysimeters in the field. Nitrogen applications were made according to field conditions and weekly petiole sampling. Irrigations were made according to field conditions and using the AZSCHED irrigation scheduling program. Treatment one was irrigated according to the schedule recommended by AZSCHED. The amount applied was equal to the total crop water use since the last irrigation. In treatment two, the timing was the same as treatment one, but the amount of irrigation water applied was 1.5 times more water. Yield samples were taken at the end of each season and showed no significant differences between treatments, with yields averaging about 1100 lb./acre of lint in 1995, 940 lb./acre of lint in 1996 and 1300 lb./acre in 1997. Cumulative drainage was 8 inches in lysimeter one and 28 inches in lysimeter two. Nitrate losses for the three years totaled 126 lb. N/acre for lysimeter two and 72.5 lb. N/acre for lysimeter one.

Water is a vital resource for cotton production in the desert Southwest. One method of managing irrigation water is through the use of a "checkbook" approach to irrigation scheduling. This involves irrigating based upon the percent depletion of plant available water (PA its from the soil profile. In order to effectively utilize this method of irrigation scheduling soil water content values at field capacity (FC) and permanent wilting point (PWP) must be defined. In this study the PWP values were characterized for two different soil types, one at Maricopa, AZ and another at Marana, AZ. The possibility of having different values for PWP as a function of crop stage of growth was also investigated in this study. Results demonstrated differences in both FC and PWP values between the two locations. Differences were also observed as a function of crop growth stage in the pattern of soil water extraction at the Maricopa location.

Irrigation scheduling is an important practice in cotton cultivation in the and southwest. AZSCHED irrigation scheduling software was used as a tool to schedule irrigation and then evaluate the soil water depletion levels over irrigation treatments with DP 90 and Pima S6. Lint yields were compared and irrigation costs were calculated for the various treatments. The highest yields and income levels came from applying irrigation when the soil depletion levels reached 40%.

Lint yield response to three irrigation treatments based on allowable soil moisture depletion regimes of 50, 75, and 100% depletion of available soil moisture was tested on both a determinate (D&PL 5415) and an indeterminate (D&PL 5816) upland cotton. Arizona Meteorological Weather Networks' (AZMET) potential evapotranspiration (ETo) estimates in combination with cotton crop coefficients were used in a summation manner until targeted depletion thresholds were reached which then triggered the desired irrigation event. The experiment consisted of three irrigation treatments with each main irrigation treatment containing both the determinate and indeterminate variety selection resulting in a randomized complete block split plot design. Actual irrigation volume delivered was 46, 42, and 32 acre inches /acre in 1994 and 52, 48, and 36 acre inches /acre in 1995 for the wet, medium, and dry treatment respectively. Lint yields were significantly reduced in 1994 when available soil moisture depletion exceeded 75% in the determinate variety with no significant yield differences in the indeterminate variety in 1994 across all irrigation treatment regimes. In 1995, lint yields were down across all treatments and varieties with the only statistically significant reduction in lint yield (relative to all 1995 yields) occurring in the dry indeterminate block

Although the cost for water is one of the largest expenditures in a grower's budget, many growers still over -irrigate their fields to assure that there will be no yield losses. Although these over -irrigations usually do not cause any negative effect to the crop, they can cause the loss of available nitrogen to the plant and the potential of nitrate contamination of groundwater resources. To assess what impact over-irrigation may have on yield and the potential for groundwater contamination, a drainage lysimeter study was initiated at the Maricopa Agricultural Center, Maricopa Arizona. Drainage lysimeters are large steel boxes with the tops open. In this study, three lysimeters were installed. The lysimeters were 80" wide (two row widths), five feet long, and six feet deep. They were placed 18 inches below the soil surface and filled with soil as to best represent the soil in its natural condition. On April 10, cotton was dry planted and watered up. Throughout the season, water samples were taken from the lysimeters and from suction lysimeters placed in the field. Nitrogen applications were made according to field conditions and weekly petiole sampling. Irrigations were made according to field conditions and using the AZSCHED irrigation scheduling program. Treatment one was irrigated according to the schedule and amount recommended by AZSCHED. In treatment two, the timing was the same as treatment one, but the amount applied was 0.25 times more. Treatment three was also irrigated at the same time but with 0.5 times more water. Yield samples were taken at the end of the season and showed no significant differences between treatments, with yields averaging about 1100 lbs /acre of lint. The drainage amounts ranged from 4" in treatment three to 1.5 inches in treatment one. The corresponding nitrate -N losses were 33 lbs/acre for treatment three and 20 lbs/acre for treatment one. Monitoring will continue over the winter to assess the impact of winter rainfall.

The tolerance of cotton cultivars to the herbicide prometryn was determined in greenhouse tests. Prometryn was applied preemergence from 0.3 to 12.0 lbs. a.i./A and postemergence from 1.2 to 12.0 lbs. a.i./A. Upland cultivars were very susceptible to injury from prometryn applied preemergence; whereas Pima S-6, Pima S-7, and Acala 1517-75 were tolerant. Postemergence treatments of prometryn applied two weeks after planting were less injurious to Upland cultivars than preemergence treatments, however, differential tolerance between, Pima and Upland cultivars was evident at the high rates of application.

Water is a vital resource for cotton production in the desert Southwest. One method of managing irrigation water is through the use of a "checkbook" approach to irrigation scheduling. This involves irrigating based upon the percent depletion of plant available water (PAK9 from the soil profile. In order to effectively utilize this method of irrigation scheduling soil water content values at field capacity (FC) and permanent wilting point (PWP) must be defined. In this study the PWP values were characterized for Iwo different soil types, one at Maricopa, AZ and another at Marana, AZ. The possibility of having different values for PWP as a function of crop stage of growth was also investigated in this study. Results demonstrated differences in both FC and PWP values between the two locations. Differences were also observed as a function of crop growth stage in the pattern of soil water extraction.

Timing of the first, post plant irrigation is a critical management decision. With the more common reduced season type varieties being grown today water management becomes increasingly important and water stress must be avoided in order to obtain optimum yields In an effort to look at the effects of the timing of the first post plant irrigation two studies were established at both the Maricopa Ag. Center and the Marana Ag. Center in 1995 and 1996. Two treatments were established based upon evapotranspiration and soil water depletion with treatment one being the optimum irrigation timing and treatment two being delayed by approximately 7-10 days. Results showed decreased plant vigor and fruit retention as a result of the delayed irrigation treatment that continued the entire season. Yield reductions associated with the delayed irrigation were also observed in all four site years, two of which were significant.

Lint yield response to varied Management Allowed Depletion Levels (MADL), and consumptive water use rates were determined for four upland cotton varieties (D&PL 5415, D&PL NuCotn 33B, D&PL 5816, and STV 474) at the Maricopa Agricultural Center. Four irrigation treatments based on MADL of 35% (Very Wet), 50% (Wet), 75% (Medium) and 90% (Dry) of available moisture were used to schedule irrigations after May 16. The Very Wet and Wet treatments showed statistically similar yields which were much greater than the Medium and Dry treatments. Irrigation return intervals of six to ten days from early June thru late July appeared to provide the greatest lint yields. Consumptive water use appears to reach its maximum during the peak bloom period of the fruiting cycle. Following peak bloom, water use gradually diminishes.

A field trial was conducted at the Maricopa Agricultural Center to observe the effects of four irrigation efficiencies (65%, 75%, 85 %, and 95 %) on the lint yield produced from two upland cotton varieties (DP 5409 and SG 125). Nitrogen requirements for the crop were determined using pre-season soil samples and in-season petiole samples in conjunction with crop monitoring data collected at weekly intervals. AZSCHED was used as a guide to the irrigation timing and amount of water applied during the season. The actual irrigation efficiencies obtained were less than what was targeted. The end season results were 60, 66, 72, and 74 %, respectively. The low efficiencies are due in part to the inherent inefficiency of irrigations in the early season. There was no lint yield response to the different irrigation efficiencies, and no difference in yield between the two varieties. Lint Yields ranged from 1146 # lint/acre (SG125 at 75 %) to 925 # lint /acre (SG 125 at the 95 % irrigation efficiency).