Irrigation Termination Thermal Time and Amount on Cotton Lint Yield and Fiber Quality

Abstract

Cotton irrigation in the Texas High Plains (THP) is often dictated by the well capacity and not by the water needs of the crop. The source of irrigation-water is the Ogallala aquifer and in many areas of the THP, the water table has declined to well capacities that deliver 1.3 to >7.6 mm/d. There is plenty of information on cotton responses to irrigation frequency and amount; however, information on when to terminate irrigation and its effect on cotton lint yield and fiber quality is scarce. Our objective was to evaluate over a 4-year period three irrigation termination thermal times corresponding to cumulative daily heat units (∑HU) of 890 ˚C, 1000 ˚C and 1110 ˚C from crop emergence, and three levels of irrigation (2.5, 5.1 and 7.6 mm/d) on cotton lint yield and fiber quality. Irrigation was applied with a sprinkler system on a 3-day frequency in Lubbock, TX. Results showed that on average the 7.6 mm/d level produced the most cotton lint yield regardless of the irrigation termination thermal time. Terminating cotton at 1000−˚C ∑HU resulted in water savings of 25 to 50 mm for the 2.5 and 5.1 mm/d levels without significantly affecting lint yield. For the 7.6 mm/d and terminating at 890−˚C ∑HU resulted in water savings of 100 to 115 mm. Average fiber length statistically increased with termination thermal time and level. This effect was most significant in years with the least rain and warmer air temperature. Micronaire increased with the termination thermal time in years with >500 mm of rain. Average fiber length uniformity and fiber strength were minimally affected by termination thermal time. As irrigation level increased, the average micronaire decreased, and fiber strength increased for the 5.1 and 7.6 mm/d irrigation. We concluded that in the THP for well capacities that deliver 2.5-5.1 mm/d irrigation can be terminated when the ∑HU reaches about 1000 ˚C from emergence without impacting cotton lint yield.

Impacts of Irrigation Termination Date on Cotton Yield and Irrigation Requirement

“…Nonetheless, the accumulated heat units (HU) prior to and after irrigation termination were estimated for each treatment to investigate the impact of thermal conditions on cotton yield. Previous studies have reported a direct influence of prevailing thermal conditions on the growth and development of cotton and recommended the use of degree heat units as a tool to make decisions about irrigation termination [24,30,34]. In the present study, variations in air temperatures, planting dates, and IT dates resulted in different HUs by each treatment amongst the three years.…”

“…These variable results demonstrate the need to further investigate the effects of irrigation termination on cotton yield. This is also evident and in support of the study by Lascano et al [24], who argued that even though there is an abundance of data on cotton yield response to the amount and timing of irrigation, very little information is available pertaining to the impact of irrigation termination timing on cotton yield and fiber quality. Vories et al [21] made the same observation, particularly for the U.S. Mid-South region, and highlighted that more research on cotton IT could help improve management practices by cotton producers, and more importantly complement water conservation efforts in arid and semi-arid regions.…”

“…The yield and fiber quality data were analyzed for each year and across the entire study period using the analysis of variance (ANOVA) at a significance level of 0.05 in SigmaPlot 14.0 [27]. To allow for pairwise comparisons among the means, the Fisher's Least Significant Difference (LSD) was also calculated and reported [24].…”

“…HVI data produces several important fiber measurements that include micronaire, fiber length, uniformity, and fiber strength. Per Lascano et al [24], micronaire is defined as the degree of fineness and maturity;…”

“…The 2015 growing season had the smallest values of micronaire compared to 2016 and 2017, and the differences in this parameter among treatments were statistically significant. A number of previous studies have highlighted the increase of micronaire with late IT, and its susceptibility to environmental conditions including rainfall and temperature [24,33]. Teague [28] observed an increase in micronaire with each additional irrigation after cutout, in a year characterized by a hot and dry mid-season in Arkansas.…”