Manure Legacy: Compost Carryover and Intercropping

McKenna Volkman^1,2, Patrick Carr^1,2, and Sally Dahlhausen^1,2 (CARC Investigators)

Jennifer Reeve³, Earl Creech³ (Project Directors)

Ian Burke⁵, Patrick Carr^1,2, Kynda Curtis⁴, Astrid Jacobson³, Man-Keun Kim⁴, Ryan Larsen⁴, Urszula Norton⁶, and Matt Yost³ (Co-Project Directors)

Montana State University, Dep. Research Centers¹; Central Ag Research Center, Moccasin, MT²; Utah State University, Dep. Plant, Soils, and Climate³; Utah State University, Dep. Applied Economics⁴; Washington State University, Dep. Crop and Soil Sciences⁵; University of Wyoming, Dep. Plant Sciences⁶

Summary                                    

Demand for organic food products by consumers means that specialty mills must source enough high-quality winter wheat to meet that need.  However, farmers face obstacles such as declining yield and quality.  The MSU Central Agricultural Research Center (CARC) is participating in a multi-state project that explores several elements of organic winter wheat systems to find methods that alleviate those challenges. Composted manure is frequently applied to organic fields as a nutrient source and soil modifier, and this trial examines the lasting effects of a single application of manure at various rates to wheat/fallow and wheat-sweetclover green fallow cropping systems.  The trial plots were established originally in spring 2021 with manure application followed by a year of substituting spring wheat for the prescribed winter wheat in rotations due to weather constraints the previous fall. The rotations were continued in fall 2021, with winter wheat being planted in the fallowed plots. Persistent drought required some modification to protocol, and spring wheat was seeded for a second year.  Grain yield and test weight, along with crop, weed, and sweetclover biomass and leaf area index (LAI) were measured and compared among plots. Due to drastic variation across plots, differences were not detected between manure treatments statistically.  However, wheat biomass and grain yield averages were larger arithmetically in wheat-only plots versus wheat/sweetclover intercrop plots (2172 lbs/ac and 37 bu/ac compared to 1042 lbs/ac and 15 bu/ac), as was LAI (1.73 compared to 1.15).  Similar to the main plot effects, no differences were detected when examining the combined effects of manure and sweetclover, and large variation occurred across plots. 

Introduction

High grain and protein yields are difficult to achieve under dryland organic management because of low levels of nitrogen that are common in many organic fields in Montana. This project investigates the effects of both intercropping and of a single composted manure application in varying rates on wheat/fallow and intercropped wheat + sweetclover green fallow rotations over a four-year period in multiple locations (CARC and Ledger in Montana, along with two sites in Utah and two in Wyoming). 

Experts in soil chemistry, agronomy, climate and weather, cropping systems, and economics from various universities are collaborating on this project to analyze many different factors.  At CARC, wheat yield and quality data are collected and analyzed, while other researchers are using soil and manure analysis and soil moisture sensors to examine soil chemical and biological content at the various sites.  

Winter and spring wheat plus intercropped sweetclover were grown in plots that were established in 2021 with differing rates of manure.  Plots were evaluated for differences based on manure application rate, presence of the intercrop, and the combined effect of both manure and intercrop. 

Methods

A split-plot design with four replications was established in spring 2021 with varying amounts of manure applied (0, 5.6, 11.2, and 22.3 tons/ac) or inorganic fertilizer (applied yearly according to soil test recommendations for 30 bu/ac wheat) as the main plot and a wheat-sweetclover green manure or wheat/bare fallow rotation established as subplots. ‘Yellowstone’ winter wheat was planted September 16, 2021 in the plots assigned to be cropped in 2022 (fallow in 2021).  No fall emergence and poor, spotty spring emergence necessitated seeding ‘Vida’ spring wheat into most plots April 7-8, 2022 at the same time that sweetclover was seeded in the intercrop plots.  Plot size was 30 by 35 feet, with destructive data collected from outer passes and grain harvest taken from the middle to allow accuracy in both.  The split plot design allows for analysis of the effect of the main plot (manure rate) on the wheat, the effect of the subplot (presence or absence of sweetclover) on the wheat, and the combined effects of both.  Plots were compared for wheat, intercrop, and weed biomass, grain yield, grain test weight, and leaf area index. 

Biomass was clipped from a 0.5 m² area, 32.3” by 24” (centered over two drill rows 12” apart), then sorted, dried, and weighed.  Grain was harvested mechanically from a 5-foot-wide pass and the length of that harvested pass measured to calculate yield accurately.  Test weight was measured using a Grain Analyzing Computer (DICKY-john) and corrected to 12% standard moisture.  Leaf area index was measured using Metergroup’s LP-80 device with external sensor to take an average of three random above + below canopy readings per plot, diagonal to drill rows. 

Results and Discussion

Main plot effects of manure application rate were not detected among any of the parameters measured (Table 1).  High variability between plots caused high CVs (23.9-206.4), so no statistical differences could be detected.  However, subplot effects were more apparent, with the wheat-only plots having a higher wheat biomass (2172 lbs/ac versus 1042 lbs/ac in the intercrop plots), wheat yield (37 bu/ac versus 15 bu/ac), and LAI (1.7 versus 1.2) with no detectable difference in weed biomass (Table 2). When examining the combined effects of manure rate plus intercrop status, results were too variable to draw firm conclusions statistically (Table 3). 

Weed pressure was extreme in 2022 throughout the trial, with winter annuals the dominant type.  This early competition likely reduced yields where weed density was greatest. The other factor that likely explained variation in grain yield and other data was interseeding spring wheat into the poor winter wheat stands. In plots where some winter wheat survived, both types of wheat contributed to yield versus plots where no winter wheat remained and only spring wheat occurred.  Ground squirrels also caused extensive damage in this trial, though attempts to harvest the “best” pass in the plot were made.  Grasshoppers moved into the trial near harvest time with lots of head clipping occurring. 

Table 1:  Main plot effects, 2022 Manure Legacy Trial, Central Ag Research Center, Moccasin, MT

NS=Not significant

¹Urea applied to each plot per soil test recommendations for 30 bu/ac wheat yield                      

²Grain yield and test weight calculated at 12% moisture

³Leaf Area Index (LAI) is the dimensionless quantity that indicates leaf area per unit of ground area, as a measure of canopy density

Table 2:  Subplot effects, 2022 Manure Legacy Trial, Central Ag Research Center, Moccasin, MT

NS=Not significant

¹Grain yield and test weight calculated at 12% moisture

²Leaf Area Index (LAI) is the dimensionless quantity that indicates leaf area per unit of ground area, as a measure of canopy density

Table 3:  Combined effects of main and subplots, 2022 Manure Legacy Trial, Central Ag Research Center, Moccasin, MT

NS=Not significant

¹Urea applied to each plot per soil test recommendations for 30 bu/ac wheat yield                      

²Grain yield and test weight calculated at 12% moisture

³Leaf Area Index (LAI) is the dimensionless quantity that indicates leaf area per unit of ground area, as a measure of canopy density