Translating Grazing: Calculating Nitrogen Credits from Cool-Season Integrated Crop Livestock Systems

Progress report for GS20-219

Project Type: Graduate Student
Funds awarded in 2020: $16,493.00
Projected End Date: 08/31/2022
Grant Recipient: University of Florida
Region: Southern
State: Florida
Graduate Student:
Major Professor:
Dr. Marcelo Wallau
University of Florida
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Project Information

Summary:

Integrated crop-livestock systems (ICLS) have been identified as having synergistic benefits in the environmental and economic facets of sustainability. Diversification of farm production, increased efficiency of land and resource uses, and soil health are among these described benefits. Despite the multitude of benefits and previous work on these systems, questions remain regarding fundamental management components and how to best translate the environmental benefits into tangible, relatable terms to agricultural producers. This project aims to address these issues by quantifying nitrogen stocks, inputs, and mineralization across a range of grazing pressures in a cover cropped, winter grazed ICLS. The data resulting from these efforts will be used to describe nitrogen dynamics and calculate potential nitrogen credits in these systems. This will serve to translate the effects of grazing cover crops in ICLS into a palpable benefit expressed in familiar terms to interested adopters of ICLS.

Project Objectives:
  1. Estimate grazing effects on nitrogen stocks and inputs in ICLS
  2. Measure soil mineralization of nitrogen during the transition from grazed cover crop to row crop phases in ICLS
    • Compare soil nitrogen mineralization under a range of grazing intensities to existing models of ungrazed cover crop nitrogen mineralization.
    • Estimate nitrogen credits provided by organic nitrogen mineralization in a range of grazing intensities.

Research

Materials and methods:

This study will evaluate the impact of grazing management of an ICL on nitrogen stocks, inputs, and mineralization. We propose to capitalize on an ongoing experiment, implemented in 2017 at the University of Florida’s North Florida Research and Education Center in Marianna, FL. We propose to build upon current activities with additional crop and soil monitoring efforts in order to further describe nitrogen stocks, inputs, and mineralization in the experiment.

Treatments and experimental design

The annual ICL rotation in the experiment supports cotton in years 1 and 2, and peanut on year 3 of the rotation. All plots (other than control) receive small grain cover crops during the cool season, plus N fertilization as described below. The treatments are organized in a randomized, complete block design with three replications and consist of a range of grazing intensities on winter cover crops (Table 1).

 

Table 1. Marianna, FL Field Experiment Treatments

Treatment

Cool-season Nitrogen Application

Grazing Intensity (Residual Grazing Stubble Mass)

Row crop, no cover crop (Control)

 

No grazing

Row crop, rye/oat cover crop

80 kg N/ha

No grazing

ICL: Row crop, rye/oat cover crop

80 kg N/ha

High (500 kg DM/ha)

ICL: Row crop, rye/oat cover crop

80 kg N/ha

Moderate (1,500 kg DM/ha)

ICL: Row crop, rye/oat cover crop

80 kg N/ha

Low (2,500 kg DM/ha)

 

Each experimental plot measures 6 x 17 m and is individually fenced. Cool-season cover crops are established in the fall of each year, no-till drilled to 56 kg/ha each of oat and rye. Nitrogen is applied three weeks after planting (34 kg N/ha) and after the first grazing event (56 kg N/ha) on high fertilization plots. Other nutrients are applied, based upon the UF-IFAS soil test recommendations. Grazing commences when herbage biomass reaches 1,120 kg DM/ha above targeted residual biomass and grazing events occur every 2 weeks thereafter. Plots are mob-stocked with 2-4 yearling steers, withdrawn from the paddocks when residual stubble mass reaches target residual biomass. Grazing time for each plot is recorded in order to estimate grazing days and stocking rate for each treatment. Both pre- and post-grazing biomass are estimated using the double-sampling procedure every 14 days during the grazing season. All crops are managed under irrigation using a central pivot system. Cash crops are planted approximately by mid-April. Crop production and harvest index are determined by harvesting a strip 1 x 2.5 m within each plot.

Response Parameters

Objective 1

Nitrogen stocks and inputs will be measured from soil and crops. Soils will be collected from each plot at three depths (0-5, 5-15, 15-30 cm) at the end of each grazing season (mid-April) and analyzed for total nitrogen, 1M KCl extractable NO3-N, and NH4-N. Cover crop contributions to nitrogen stocks and inputs will be measured in below and aboveground biomass. Root biomass and growth dynamics will be measured in three samplings using a 10.8×20 cm soil corer before initiating grazing, approximately six weeks into the grazing season, and at the end of the grazing period. Simultaneously, ingrowth of cover crop roots will be measured using wire and mesh 7.5×30 cm cylinder root ingrowth cores as described by Cooley et al. (2019). Two separate sets of these ingrowth cores will be deployed serially for approximately six weeks buried in the soil at initiation of grazing, removed six weeks later and another set deployed, then retrieved at the end of the grazing season. Roots from each of these assessments will be washed from their respective soil sample, dried, and weighed. Composited dried root samples from all depths within a plot will be analyzed for total nitrogen. Aboveground biomass production will be measured by the forage monitoring efforts previously described. Forage samples from each plot will be characterized on a variety of chemical parameters including nitrogen using NIRS (near-infrared spectroscopy).

 

Objective 2

Relative nitrogen mineralization rates (release of plant available nitrogen) will be measured during the transition from cover crop to row crop phases of the system to compare relative availability of mineralized nitrogen from grazed cover crops using the ion-exchange membrane technique (Qian & Schoenau, 2005). Strips of dimensions 2.5×10 cm of anion and cation exchange membranes will be used. Four pairs of strips (anion and cation) will be placed inserted into the soil in each plot. Ion-exchange membrane strips will be installed starting at cover crop termination and be retrieved and replaced every four weeks until week 12. Upon completion of each interval strips will be removed from soil, cleaned and NO3-N and NH4-N will be extracted from them with 1M KCl. Resulting extractants will be colorimetrically analyzed for NO3-N and NH4-N.

Relative mineral nitrogen availability measured by ion-exchange membranes will be compared against modeled nitrogen credits using the University of Georgia cover crop nitrogen availability calculator (Woodruff et al., 2018). This comparison will serve to potentially verify the utility of this model for grazed cover crops where the animal component likely alters the nitrogen mineralization dynamics and to gauge if any tradeoffs in nitrogen availability result from different levels of grazing intensity.

Research results and discussion:

The first season of field activities for this project was initiated in December of 2020. The first set of root biomass core samples were collected and the first round of root ingrowth cores were installed on December 11, 2020. A second set of root biomass cores samples and ingrowth cores were collected and a new set installed on January 28, 2021. The cover crop is to be terminated and the next set of root ingrowth cores to be collected along with root biomass cores in mid-April.

Soil sampling for nitrogen availability and the ion-exchange membrane nitrogen availability components of the study will be implemented starting in mid-April.

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

The pending results of this project are planned to be compiled and disseminated through a UF-EDIS fact sheet, annual extension agent agronomy trainings, the annual cool-season forage field day (should COVID restrictions permit), a trade journal article, a presentation at the 2021 agronomy society of America conference, as well as through multimedia in the UF forage team social media and Youtube channels.

Project Outcomes

1 Grant received that built upon this project
Project outcomes:

Metrics of project outcomes will include implementation of cool-season cover crops and grazing by cash crop farmers, implementation of cool-season forage grazing best management practices by farmers, as well as crop rotation diversification through introduction of cool-season forages into crop rotations.

Knowledge Gained:

To date through this project and other’s, we have grown a greater appreciation for the need for maintaining the practical relevance of sustainable agriculture research efforts to the needs and reality of the challenges facing producers in implementing more sustainable practices.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.