Date of Award

8-2018

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Agronomy

Committee Chair

Shaun Casteel

Committee Member 1

Jim Camberato

Committee Member 2

Kiersten Wise

Abstract

Soybean [Glycine max (L.) Merr.] management strategies have become more intensive in recent history in attempt to maximize yields, but how these treatments influence yield components is not well understood. Soybean yield can be simplified into the number of seeds per area and seed size. Seed size is the product of seed growth rate and seed fill duration. The objectives of this research were to determine the effects of agronomic management, through fertilization and foliar application of fungicide and insecticide, across cultivars from 3 decades (1970, 1990, and 2010) on seed growth and pod distribution throughout the soybean canopy. Six varieties were selected from maturity groups (MG) II and III, with two varieties chosen from circa 1970s, 1990s, and 2010s from each MG study. These varieties were grown in high fertility soil near Lafayette, Indiana in 2016 and 2017 and subjected to six agronomic management treatments. Intensely sampled treatments were an untreated check (UTC), an application of N, P, and S fertilizer (TSP+AMS), and a R4 fungicide (fluxapyroxad and pyraclostrobin) + insecticide (lambda-cyhalothrin) application in combination with the fertilizer application (TSP+AMS+R4). Seven in-season biomass harvests occurred approximately every seven to ten days beginning at R4 (full pod) and ending at R8 (full maturity). A half meter from a bordered row was sampled at each prescribed growth stage. Following harvests, plants were partitioned into stems, leaves, pods, and seeds at R4 and R6 and at the other timings into stover, seeds, and pods. The number of pods on each node were recorded for all plants in the R8 (full maturity) samples to determine the pod distribution among the plant canopy.

Agronomic treatments consisting of enhanced fertility alone or in combination with foliar fungicide and insecticide increased reproductive responses to increase yield on select parameters across MGs and years. Varietal selection had strong impact on the majority of the parameters measured. In 2016, the MG II seed growth rate (SGR) averaged over varieties and management treatments was 20.6 g m-2 d-1 over the period of 24 d (UTC) to 29 d (TSP+AMS+R4). The MG II study in 2017, produced an average SGR of 22.8 g m-2 d-1 over all treatments that lasted 24 d for P28T33 to 30 d for Wells. In 2016, the MG III study produced an increasing SGR across agronomic management treatments: UTC < TSP+AMS < TSP+AMS+R4 (16.0, 18.1, and 20.3 g m-2 d-1, respectively). For the MG III study in 2017, SGR was similar for all agronomic managements but different across varieties, ranging from 16.7 g m-2 d-1 (Thorne) to 24.7 g m-2 d-1 (P34T07) over the course of 26 d.

Averaged over all treatment combinations, starting at the bottom of the plant and moving up, nodes cotyledon-3 held 5.6% of all pods, nodes 4-8 had 32.6%, nodes 9-13 had 40.5%, nodes 14-18 had 19.7% and nodes 19 and above had 1.6% of pods in MG II. Maturity group III had similar results with 4.8, 25.6, 40.8, 25.5, and 3.4% found between nodes cot-3, 4-8, 9-13, 14-18, and 19 and above, respectively, of the plant canopy. Across both years, Beeson (MG II, 1968) produced the most nodes, pods, and seeds, as well as the largest seeds to yield the highest. P34T07 (MG III, 2014) generally maximized yield components in both years to yield the greatest. Averaged over the MG II varieties, the TSP+AMS+R4 agronomic management treatment increased the number of nodes, pods, and seeds plant-1 as well as had a heavier seed weight and yielded more than the other treatments. No consistent pod or seed increases were found in the MG III agronomic management treatments, with an average of 38 pods plant-1 in 2016 and 40 pods plant-1 in 2017.

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