Probabilities of having minimum amounts of available soil water at wheat planting

Authors: Nielsen, David; Vigil, Merle

Submitted to: Field Crops Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/9/2018
Publication Date: 2/23/2018
Publication URL: http://handle.nal.usda.gov/10113/5913900
Citation: Nielsen, D.C., Vigil, M.F. 2018. Probabilities of having minimum amounts of available soil water at wheat planting. Field Crops Research. 221:32-37. doi:10.1016/j.fcr.2018.02.008.

Interpretive Summary: Winter wheat-fallow remains a prominent cropping system in the Central Great Plains even though the second summer fallow period has been shown to be highly inefficient at storing precipitation. A second reason that wheat fallow continues to see a lot of use may be because reducing the frequency of fallow or eliminating fallow altogether may reduce available soil water at planting. Because wheat yield is highly resonsponsive to available soil water at planting, wheat yields could therefore be potentially reduced when using more intensive cropping systems. Using soil water data from a 25-yr study conducted at Akron, CO on a silt loam soil, we found that average available soil water at planting was unaffected by reduction in fallow frequency for wheat grown in either no till or reduced till systems as long as the fallow period before wheat planting was 12 to 14 months long. However, if wheat was grown using conventional till practices, averaged available soil water at wheat planting was reduced by 81 mm (3.2 inches) to a value of 156 mm (6.1 inches). If the fallow period before wheat planting was eliminated, the average available soil water at wheat planting was further reduced to 100 mm (3.9 inches). Using our long-term data set we constructed cumulative probability exceedance graphs that allow farmers to determine the probability of having given amounts of minimum available soil water at planting. These graphs provide a valuable decision support tool for helping farmers assess risk in moving from a wheat-fallow cropping system to a system with greater cropping intensity and diversity.

Technical Abstract: Winter wheat (Triticum aestivum L.)-fallow (WF) remains a prominent cropping system throughout the Central Great Plains despite documentation confirming the inefficiency of precipitation storage during the second summer fallow period. Wheat yield is greatly influenced by available soil water at planting, and that quantity will be influenced by cropping intensity, tillage intensity, crop grown prior to winter wheat planting, and length of the non-crop period prior to winter wheat planting. The objective of this study was to determine the effect of cropping system intensity, tillage, and crop prior to wheat planting on available soil water at planting and the probabilities of having certain minimum amounts of available soil water at wheat planting. Available soil water was measured at wheat planting for nine cropping systems varying in fallow frequency, tillage intensity, and crop prior to wheat over a 25-yr period at Akron, CO. Available soil water at wheat planting was found to be greatest for NT and RT systems in which a fallow period of 12 to 14 months preceded wheat planting. The lowest amounts of available soil water at wheat planting were found for the continuously cropped rotations. The cumulative probability exceedance graphs of available soil water at planting fell into three similar groupings: 1) the five rotations which had a no till or reduced till fallow period prior to the next wheat crop; 2) the WF(conventional till) rotation; 3) the three rotations with either pea (Pisum sativum L.) or millet (Panicum miliaceum L.) prior to the next wheat crop. These graphs provide a valuable decision support tool for helping farmers assess risk in moving from a WF cropping system to a system of greater cropping intensity and diversity.