Date of Award:

8-2020

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Plants, Soils, and Climate

Department name when degree awarded

Plant Soils and Climate

Committee Chair(s)

Matt Yost

Committee

Matt Yost

Committee

Grant Cardon

Committee

J. Earl Creech

Abstract

Wheat, barley, triticale, and oats, are small grains commonly grown as hay and grain following alfalfa in Utah and the Intermountain West, especially during drought years as they require less irrigation than corn. Several studies in many parts of the world have shown that first-year corn following alfalfa rarely needs nitrogen (N) fertilizer, yet relatively few have evaluated the N needs of small grains, especially small grains grown for hay. Objectives of this research were to determine whether N fertilizer is needed to economically optimize the yield and quality of first-year small grains following alfalfa, develop N guidelines, and whether spring soil nitrate or leaf chlorophyll concentrations at flag leaf can predict N response. Fertilizer trials were conducted at 30 different locations in the first-year following alfalfa during 2018-2019 in Idaho, Utah, and Colorado. At each location, up to 13 different N treatments ranging from 0 to 168 kg N ha-1 were applied as ammonium nitrate in the fall, spring, or mid-season. Results indicate that for small grains grown to kernel maturity the alfalfa N credit was not adequate to increase levels in most cases, yield (56%), test weight (33%), and protein (83%). Out of all responsive locations, 93% had a spring soil nitrate level lower than 21 g kg-1, indicating that spring soil nitrate may play a role in responsive locations. Responsive locations required up to 115, 108, and 148 kg N ha-1, in yield, test weight, and protein, respectively. Spring soil nitrate and leaf chlorophyll concentration tests were able to accurately predict grain yield response to N in 53% and 17%, respectively, and for grain quality in 80% and 64%, respectively. These results suggest that growers who have a spring soil nitrate level less than 21 g kg-1 may still apply up to 115 kg N ha-1 in the spring. Doing so can increase yield up to 31%, test weight up to 1.37%, and protein up to 20%. For hay, results indicate that N fertilizer was not needed to economically increase yield at most (91%) locations. The one responsive location following an old alfalfa stand (> 9 yr) and required only 67 kg N ha-1 to economically optimize yield. In contrast, hay quality improved at nearly all locations with N fertilizer applications up to 112 kg N ha-1. Soil nitrate prediction tests were able to separate yield response to N in 45% of the cases, and were able to separate hay quality response to N in 67% of the cases. This suggests that leaf chlorophyll concentrations and soil nitrate may be viable prediction tests to determine yield and quality responses in this rotation. These results indicate that growers may be able to withhold additional N fertilizer depending on the following conditions: if the small grain crop is harvested for grain or for hay forage, if soil nitrate levels are lower than 21 mg kg-1, if small grains follow an older (10+ yr) stand, also if compensation for small grain quality improvement outweighs fertilizer cost. These are key factors in determining additional N fertilizer need when growing small grains in the first-year after alfalfa. This information will help grower's better utilizer N credits from alfalfa, improve their small grain yield, quality, profits, and reduce negative implications of excessive N fertilizer applications.

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