Date of Award:

8-2021

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)

Grant Cardon

Committee

Grant Cardon

Committee

Earl Creech

Committee

Paul Grossl

Abstract

Silage corn (Zea mays L.) following alfalfa (Medicago sativa L.) is a common crop rotation in Utah and southern Idaho. This is done, in part, to take advantage of residual nitrogen (N) fixed by bacteria that work in symbiosis with the roots of alfalfa. After alfalfa is terminated, much of the N that was fixed by the plant is released into the soil and becomes available for use by the rotational crop. This reduces the amount of N fertilizer that growers need to apply. The Utah Fertilizer Guide currently recommends an N credit to a rotational crop following alfalfa of 112 kg N ha-1. On-farm experiments were conducted to test whether this credit is valid and to what extent residual N from a previous alfalfa crop benefits corn silage yield and quality in the first- and second-year corn following alfalfa. Four N rates were tested in this experiment (0, 56, 112, and 224 kg N ha-1). The data from 27 site-years of first-year corn showed that yield increased from 22.1 to 23.1 Mg ha-1 as the nitrogen application increased from 0 to 224 kg N ha-1. The economic optimum nitrogen rate (EONR) was lower than the currently accepted recommended rate on all 27 first-year sites. Nine site-years of second-year corn showed that yield also increased 19.8 to 21.8 Mg ha-1 as application rate went from 0 to 224 kg N ha-1. Eight of the nine sites had an EONR of 0 kg N ha-1. A better understanding of how residual nitrogen affects crop growth will improve nitrogen recommendations and reduce farm costs for rotational crops following alfalfa.

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