| Summary: | Field experiments were conducted in 2004 and 2005 at the University of Arizona Maricopa Agricultural Center (1,175ft. elevation) to evaluate the effects of five irrigation termination (IT1, IT2, IT3, IT4, and IT5) dates on yield and fiber micronaire of eleven Upland cotton varieties and one Pima variety. In addition, the economic relationships of IT treatments were also evaluated. The experimental design was a split plot in a randomized complete block design with three replications. The main treatments included the five IT dates and the subunits consisted of 11 Upland varieties and a Pima variety. The first two IT treatments (IT1 and IT2) were imposed with the intention of terminating irrigations very early and pre-maturely at peak bloom. Based upon current UA recommendations for IT to complete a single cycle fruit set, the more optimal date of IT would have included one or two additional irrigations (beyond IT1 and IT2). In this experiment, IT2 was structured to provide an additional (one) irrigation just past peak bloom. For the IT3 plots, the intention was to attempt to time termination in advance of cutout. The 2004 and 2005 IT4 and IT5 were imposed to attempt to complete the primary fruiting cycle development and produce a second cycle fruit set that require irrigations until late August and late September, respectively. In general, lint yield and micronaire results revealed significant differences among the IT treatments and varieties. In a similar fashion to a previous set of IT experiments (2000-2002), lint yield and micronaire values consistently increased with later IT dates. The best combined lint yield and micronaire results were achieved with IT4 date, which received 12 and 18 in. less irrigation water than IT5 in 2004 and 2005, respectively. In 2004 and 2005, the 12 and 18 in. water saved equate to approximately 20% and 30% less water used under the conventional practice, respectively. The average marginal value of water for all eleven Upland varieties in going from IT1 to IT2, IT2 to IT3, IT3 to IT4, and IT4 to IT5 for November 2004 prices and low carrying costs is calculated at $320.07, $150.15, $100.54, and -$28.16 per acre-foot of water. If steeper mike discounts (November 1999), a lower base lint price (45¢/lb.), and higher costs (i.e., more costly insecticide and chemical costs) are imputed to extend the crop, the marginal value of an acre-foot of water for all Upland varieties and replications in going from IT1 to IT2, IT2 to IT3, IT3 to IT4, and IT4 to IT5 is estimated at $164.04, $48.15, $12.97, and -$94.79. Profitability and the value of water for extending the season varies quite markedly between different varieties and termination dates.
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