Irrigated Spring Wheat Response to Topdressed Nitrogen as Predicted by Flag Leaf Nitrogen Concentration

Abstract

Topdressed N is frequently applied to irrigated hard red spring (HRS) wheat (Triticum aestivum L.) in an effort to increase grain protein content (GPC). However, the actual need for supplemental N could be better defined if the probability of a response was referenced to some measure of plant N status. This study was conducted to determine whether flag leaf N (FLN) concentration at heading could be used to estimate the potential for increasing GPC with topdressed N. Field studies were conducted in 1989, 1990, and 1991 on a Declo silt loam (coarse loamy, mixed, mesic Xerollic Calciorthid). Two HRS wheat varieties (Copper and Pondera) received preplant applications of either 0, 60, 120, or 180 lb N/acre to produce a range of FLN concentrations at heading. Plants at each N level were top-dressed with either 0, 20, or 40 lb N/acre 1 to 2 d after heading. Topdressed N had relatively little effect on grain yield or test weight. Flag leaf N concentrations and GPC were both increased by preplant and topdressed N applications, however. Grain protein contents of both varieties were positively correlated with N concentrations of flag leaves sampled prior to N top-dressing. Grain protein content of low N plants (3.0 to 3.6% N) was increased by 1.0 to 1.5% with 20 lb N/acre topdress applications and 2.0 to 2.5% with 40 lb N/acre applications. Increases in GPC resulting from N topdressing decreased linearly as FLN at heading increased. Plants with FLN concentrations above 4.2% did not benefit from topdressed N. Results of this study provide preliminary evidence that FLN concentration can provide a good indication of the likelihood of a response to topdressed N. Research Question Irrigated hard red spring wheat growers are encouraged to split N applications during the growing season to increase grain protein content (GPC) and N-use efficiency. Split N applications could be managed more effectively if there was a predictive tool available to correlate plant N status with the potential for a GPC response to N topdressing. The primary objective of this research was to evaluate flag leaf N (FLN) concentration as a more precise predictor of GPC response to N topdressing. Literature Summary Studies conducted with irrigated and nonirrigated wheat production systems have shown that wheat GPC can be influenced by late split N applications. Responses are especially effective when application is made between heading and flowering. The response becomes limited, however, if plant N status is high. Pacific Northwest wheat growers frequently apply late-season N fertilizer in hopes of obtaining a GPC response. Few studies have been completed which provide growers with information to predict a response to these late N fertilizer applications. Study Description Two irrigated hard red spring wheat cultivars were planted in combination with four preplant and three topdressed-N fertilizer rates in each of 3 yr at Aberdeen, ID. Wheat varieties: Copper and Pondera, 110 lb/acre in 7 in. row Soil: Declo silt loam Fertility: Idaho Soil Test Irrigation: Sprinkler irrigation Weed control: Bromoxynil and MCPA Treatments: N was applied preplant at either 0, 60, 120, or 180 lb N/acre Topdressed N rates of 0, 20, or 40 lb N/acre were applied at heading N fertilizer: Ammonium nitrate (34-0-0) Flag leaf N sampling: Heading (Zadoks 59) and early milk (Zadoks 73) Sample size: 25 flag leaves/plot Applied Question Is FLN concentration at heading a good predictor of GPC response to topdressed N in irrigated hard red spring wheat? When hard red spring wheat plants were topdressed with N at heading, there usually was an increase in GPC. This increase was closely related to FLN concentration at heading. Nitrogen topdressing increased GPC of low N plants (3.0 to 3.6% N) by 1.0 to 2.5%. As FLN at heading increased, the GPC response to topdressed N became progressively smaller. Plants with FLN concentrations above 4.2% generally did not respond to topdressed N. Heading is an appropriate sampling time since topdressed N is often applied during flowering. The relationships between FLN and GPC may differ between locations, soil moisture conditions, and other environmental factors. Therefore, local calibration of FLN relationships with yield and GPC probably will be required for specific production areas. Recommendations We recommend that growers who have a desire to increase GPC with late season N applications use FLN concentrations at heading as an appropriate predictor for response.