Submitted to: Open Journal of Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/8/2017
Publication Date: 9/11/2017
Citation: Lascano, R.J., Baumhardt, R.L., Goebel, T.S., Baker, J.T., Gitz, D.C. 2017. Irrigation termination date and amount on cotton lint yield and fiber quality. Open Journal of Soil Science. 7:216-234 http://www.scirp.org/journal/ojss. Interpretive Summary: Crop irrigation involves answers to questions on when to start and when to stop irrigation, and on how much water and when should the irrigation-water be applied. In the Texas High Plains (THP) answers to these questions are specific to the amount of water each producer has to irrigate the crop and this is a function of the well capacity. The water table of the Ogallala Aquifer, the source of the irrigation water, has declined to the point that the amount of water available to irrigate is less than the environmental demands of the crop. There has been a lot of research on irrigation frequency and amount but information on when to stop irrigation is scarce. We wanted to answer the question on when to stop irrigating cotton in the THP. For this purpose and over a four-year period we did a field experiment with cotton where we examined three different dates of termination for three irrigation levels of 0.1 (low), 0.2 (medium) and 0.3 (high) inches per day. The irrigation termination dates were based on the accumulation of daily heat units (HU) from when the crop emerged. The daily HU was based on the threshold value of 60 F and we selected cumulative HU (SHU) values of 1,600 (early), 1,800 (intermediate) and 2,000 (late) F. The experiment was done in Lubbock, TX using a randomized statistical design, split by the three irrigation levels and split by three-irrigation termination dates, and each treatment was replicated four times. The crop was irrigated with a LEPA system and water was applied on a 3-day frequency. Results showed that when a producer has a high amount of water they could stop irrigating at the early SHU and for the low and medium level they could stop irrigating at the intermediate SHU without affecting the cotton lint yield. Important variables to consider on when to stop irrigation is the amount of rain and HU during the growing season as the collectively affect lint yield. The effect of the irrigation termination date on the quality of the fiber is more apparent in dry and warmer years.
Technical 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 dates corresponding to cumulative daily heat units (SHU) of 890 ºC, 1,000 ºC and 1,110 ºC 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 date. Terminating cotton at 1,000–ºC SHU resulted in water savings of 25 to 50 mm for the 2.5 and 5.1 mm/d levels without statistically affecting lint yield. For the 7.6 mm/d and terminating at 890–ºC SHU resulted in water savings of 100 to 115 mm. Average fiber length statistically increased with termination date and level. This effect was most significant in years with the least rain and warmer air temperature. Micronaire increased with the termination date in years with > 500 mm of rain. Average fiber length uniformity and fiber strength was minimally affected by termination date. As irrigation level increased the average micronaire decreased and fiber strength increased for the 5.1 and 7.6 mm/d irrigation. We conclude that in the THP for well capacities that deliver 2.5 – 5.1 mm/d irrigation can be terminated when the SHU reaches about 1,000 ºC from emergence without impacting cotton lint yield.