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Fluid Journal : Fluid Journal 1993-1995
2 Fluid Journal Fall 1993 Table 1. Nitrogen requirements for various yield levels, depending on water supplied. Water Supply Lint Yield Seed Yield Total Biomass N in Plant ----- - - - -lbs/A- - - ------ Dryland 200-500 300-800 2,000-3,000 30-65 (8-12") Limited Irrigation 500-800 700-1,250 3,000-6,000 60-125 (14-18") Full Irrigation 800-1,200 1,150-1,200 5,000-8,000 125-250 (20-24") Table 1. Cotton yield response to irrigation frequency and volume. --------IrrigationFrequency-------- % ET replace 3 days 6 days 12 days 18 days ------ - - - -lbs/A* - -------- - .04 850 875 780 700 .06 1,050 1,080 950 750 .08 1,100 1,120 1,050 790 1.00 1,000 1,020 1,000 820 Average 1,000 1,024 945 765 *Average across 3 years. 1.0 ET = 0.25 inches/day averaged across growing season. Soil at field capacity in top three feet prior to planting. Table 1. Yield and yield components of cotton produced at two populations within two different water supplies--dryland (12 inches) and irrigated (20 inches). 12 inches water Plant 1st 2nd Density Pos. Bolls Pos. Bolls Yield plants/A No. Size No. Size (lbs/A) (g) (g) 40,000 5 1.2 1 1.1 625 80,000 2 1.1 --- --- 321 20 inches water 40,000 7 1.4 2 1.2 1,284 80,000 4 1.2 1 1.0 974 Water requirements. The real question is: How much water is required to grow ground cover? Current estimates are at least 3 to 3.5 inches to termination. An additional 1 to 2 inches are lost due to the "wick effect" after the wheat ground cover has been chemically terminated. Average precipitation from November through March is usually only 0.5 inch/month and is highly unreliable. Water for seedling establishment and early spring growth is required to get the required benefits from this system---thus irrigation (center pivot preferred) is essential for success. If water returns are not equivalent, however, the use of 4 to 5 inches of this very precious water to grow wind protection in this semi-arid region may not be justified. A reduction in soil E with a concomitant increase in T and no difference in total ET has been documented for the growing season water use, resulting in yield increases. However, yield increases have not been consistent in all studies. Dryland conditions. The wheat cover system simply will not work under rain- fed (dryland) conditions due to the uncertainty of precipitation during the fall for establishment and insufficient precipitation during early spring when growth resumes. Under dryland conditions, two options are available to minimize the potential damage of plant water stress. One strategy fits a variety's growth habits to cultural practices. A slightly more indeterminate growth habit is preferable under dryland conditions whereas a slightly more determinate growth habit is preferred under irrigated conditions. The indeterminate growth habit allows the plant to suspend reproductive development during the stress period but resume development when rain occurs. If the growing season is sufficiently long, good yields can be achieved. However, if the fall is wet and cool, not only will yields be low but fiber quality will also be very poor. The other option is to optimize plant density relative to water supply. We have determined the water requirement per plant to reach flowering without suffering water stress. Based upon the plant-available volume of stored soil water at planting and the probability of a given volume of rainfall during the next 60 days, seeding rates can be adjusted to provide a final stand that will not limit yield if rainfall exceeds expected levels nor will it suffer excessive stress if rain-fall is less than expected. We have based the water requirements per plant on one-half the genetic yield potential of the cotton plant to allow flexibility in the production capacity of the plant. In most years, the soil profile is only 50 to 60 percent full at planting and the probability for 3 to 4 inches of rain
Fluid Journal 1996-1998