Continuous winter wheat response to nitrogen fertilizer rate in long-term reduced tillage semi-arid variable yield environments

Reduced tillage (RT) increased soil water storage compared with conventional tillage, enabling an associated increase in crop performance. However, information on nitrogen (N) fertilizer rates for RT systems is limited. The objectives of this long-term study were to determine agronomic and economical optimal N rates for a continuous winter wheat (Triticum aestivum L.) system under RT at different yield environments. This study was conducted at Kansas State University Agricultural Research Center near Hays, KS, from 1971 through 2003. There were six N fertilizer rate treatments (0, 22, 45, 67, 90, and 112 kg N ha-1) arranged in a randomized complete block design with six replications. Over the 33 years, winter wheat yields ranged from a minimum of 370 kg ha-1 to a maximum of 3948 kg ha-1 with mean yield of 2015 kg ha-1 and standard deviation of 669 kg ha-1. Agronomic optimal N rates for wheat varied from 70 in very low yield (VLY) to 79 kg N ha-1 in very high yield (VHY) environment. The economic optimal N rates estimated for continuous wheat ranged from 59 in VLY to 71 kg N ha-1 in VHY environments. However, net profits from fertilizer application ranged from $63 in VLY to $443 ha-1 in VHY environment. We concluded that applying 60-70 kg N ha-1 would optimize dryland continuous RT wheat yields and profitability, but N removal and recovery efficiency (grain N [applied N]-1) are less in lower yielding environments. Agronomic optimal N rates ranged from 70 to 79 kg N ha-1 across yield environments.Yield at agronomic optimal rate significantly varied across yield environments and ranged from 1246 to 3332 kg ha-1.The estimated economic optimal N rates ranged from 59 to 71 kg N ha-1 across environments.Revenue at economic optimal N rate varied by yield environments and ranged from $63 to $443 ha-1.