Agroecological Intensification of Warm-season Pastures for Improved Productivity and Quality and Ecosystem Services

Progress report for GS20-222

Project Type: Graduate Student
Funds awarded in 2020: $16,173.00
Projected End Date: 02/28/2023
Grant Recipient: University of Florida
Region: Southern
State: Florida
Graduate Student:
Major Professor:
Chris Wilson
University of Florida
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Project Information

Summary:

Bahiagrass pastures (Paspalum notatum Flugge; BG) dominate Florida grazing land. Grown on over one million hectares, this warm-season perennial grass produces abundant, low-quality forage. Seasonal dormancy and low soil fertility limit BG production and require feed and fertilizer inputs to support forage production and quality. However, high input costs and scrutiny of the carbon footprint associated with fertilizer and feed production and transport highlight the need for sustainable alternatives. Integrating legumes and over-seeding cool-season annual forages into BG pastures supplies N through biological N2-fixation while cool-season annuals extend the grazing season, thus reducing the need for fertilizer N and stored feed. Species diversification of BG pastures over time and space, referred to here as agroecological intensification (AEI), may enhance soil organic matter (SOM) and improve nutrient cycling by changing the quality, quantity and timing of organic inputs. Despite advances in mixed BG-rhizoma peanut (Arachis glabrata Benth; RP) research, the influence of RP proportion and other forage species on below-ground responses linked to SOM formation and nutrient cycling remains understudied. This project aims to evaluate increasingly intensified pasture systems for forage productivity and quality and to understand root-rhizome and microbial outcomes of intensification and pasture composition. Ultimately, it seeks to maximize pasture productivity, promote on-farm resource use efficiency and enhance ecosystem services by supporting SOM and year-round plant growth to reduce external inputs.

Project Objectives:
  1. To evaluate the productivity and quality of forage systems along a gradient of AEI. Specifically, we will examine both overall productivity and quality as well as the smoothness of their seasonal distribution (i.e. ability to reduce seasonal over/under-supply), given the large importance of the latter to realizing production benefits in practice.
  2. To evaluate root and rhizome production along a gradient of AEI, and to link these below-ground responses to SOM formation and soil fertility indicators.
  3. To explore how variations in legume proportion, soil quality (indexed by baseline SOM) and soil resource supply (including fertility and soil moisture) relate to variations in the realized production and ecological benefits of AEI.

Research

Materials and methods:

Experimental design: Forty-eight 1.4-m x 1.4-m experimental plots were established at the Beef Research Unit in Gainesville, Florida in an existing  system of experimental plots of 'Pensacola' BG-dominated and BG-RP pasture. Bahiagrass-RP contributes to plant diversity within a given pasture by growing two warm-season forages concurrently. The 'Ecorturf' RP variety was selected for its high nutritive value, grazing tolerance and persistence. Over-seeded cool-season annual forage treatments add a temporal diversity dimension by growing multiple species at different times. Cool-season species were selected based on previous research and life cycle: rapid germination of rye and oat provide early cool-season growth, followed by annual ryegrass peaking in January, and late emergence of red clover (Fonatelli & Sollenberger, 2000). Per common practice, we will supply cool-season annual forages with 18 kg/ha of N fertilizer. To maximize the real-world relevance of our results, and to isolate N-related from non-N-related benefits of AEI, we include both unfertilized and N fertilized bahiagrass (Bahiagrass-N, 2 split doses of 40 kg N/ha) as treatments.

A completely randomized block design with blocks of warm-season perennials overlaid with a cool-season annual forage treatment will consist of:

1) Bahiagrass

  1. without cool-season grasses
  2. with over-seeded cool-season grasses + N fertilizer
  3. with over-seeded cool season grasses + N fertilizer + legume

2) Bahiagrass-N

  1. without cool-season grasses
  2. with over-seeded cool-season grasses + N fertilizer
  3. with over-seeded cool season grasses + N fertilizer + legume

3) Bahiagrass-Rhizoma peanut

  1. without cool-season grasses
  2. with over-seeded cool-season grasses + N fertilizer
  3. with over-seeded cool season grasses + N fertilizer + legume

4)  Bahiagrass-N - Rhizoma peanut

  1. without cool-season grasses
  2. with over-seeded cool-season grasses + N fertilizer
  3. with over-seeded cool season grasses + N fertilizer + legume

All plots will be maintained with simulated grazing. Based on previous research, we determined that a 4-wk frequency of defoliation to 5 cm represents a decent compromise to simulate grazing utilization while maintaining adequate persistence and quality in both BG and RP (Stenklyft, 2017; Mislevy et al., 1991).

Objective 1:

            To quantify pasture productivity, quality and their seasonal distribution at each level of AEI, aboveground biomass samples will be collected monthly from each plot using a 0.0625 m2 quadrat. Measures of monthly fresh and dry biomass weights summed together amounts to the total forage production of each plot. Forage tissue samples analyzed for crude protein and in vitro digestible organic matter will measure forage nutritive value. Total production and quality as a function of position on the AEI gradient will be analyzed with mixed-effects linear regression, while seasonal distribution will be quantified using measures of dispersion and Fourier series analysis. Taken together, this information will illustrate the productive potential of AEI pastures compared with conventional BG pastures with and without N fertilizer and combined with our forage quality measures will inform on the potential to improve animal production and profitability.

Objective 2:

To quantify below-ground root and rhizome production and link them to changes in SOM and microbial biomass at each level of AEI, we will combine sampling of soil cores, with root in-growth core methods.

Changes in SOM and soil microbial biomass will be measured by collecting soil cores prior to the initiation of treatment implementation and at the close of the experiment. Since bulk SOM often takes many years to change appreciably in response to management interventions, size-density fraction will be used to separate pools of organic matter that turn over on different timeframes. In particular, we will use the fast-turnover light coarse fraction as a leading indicator for changes in SOM. Soil analysis using an elemental analyzer will quantify total soil C and N content, while a modified slurry chloroform fumigation extraction (Fierer et al., 2003) will quantify microbial biomass, which responds very quickly to management.

While soil cores give useful snapshots of the below-ground ecosystem, on their own they cannot be used to measure below-ground production since turnover is not accounted for. For this purpose, we will collect three in-growth cores per plot per year corresponding to early warm-season (May), late warm-season (September), and peak cool-season (January). In each extracted core, we will separately weigh roots and rhizomes and analyze for total C and N content and δ13C using elemental analysis. The isotopic δ13C from cores in mixed plots will enable partitioning of the observed biomass into a C4 BG component and a C3 RP component.

Soil and microbial C and N will be measured from root in-growth cores using the above methods. We will then analyze the relationship between below-ground production and changes in our leading indicators of microbial and soil light fraction C and N using linear regression while accounting for treatment and block.

Objective 3:

Separate from our analyses of the impacts of AEI treatment, we will examine how pre-existing, underlying differences in plot characteristics (e.g. SOM, fertility, average soil moisture, proportion of RP) impact variations in the realized production and below-ground benefits of our treatments. This will provide useful and novel insight not only to extend the general framework of AEI, but also will be directly actionable information for farmers and ranchers.

We will measure proportion of legume in the above-ground biomass samples described in Objective 1 by hand separation. Meanwhile, root-rhizome δ13C isotope results described in Objective 2 will be used to inform on below-ground proportions from C3/C4 species. Linking information about the ratio of RP to BG to the information obtained in Objectives 1 and 2 can guide management by targeting specific RP proportions based on expected productive and ecological outcomes.

Plot level SOM and fertility will also be collected as described in Objective 2. Moreover, because our experimental paddocks are blocked along a gradient of soil moisture, weekly soil moisture measurements will be taken using a Hydrosense sensor to 11-cm depth. These characteristics will be used to explain variations in both total forage production, quality and their seasonality using linear regression techniques. The insight gained here will assist scientists and farmers in predicting the benefits from AEI strategies, and guide management in how to optimize them.

Research results and discussion:

To date, we have completed the first year (Sept. 2020-Aug. 2021) of data collection and processing and currently second year (Sept. 2021-present) data collection and processing are underway. Data analysis is in its initial stages and will be a primary focus during the remainder of this project. Below are a couple of examples of data visualizations produced from year 1 data.

 

Fig
Figure 1. Weekly soil moisture measurements from September 2020 through the present.
Fig 1
Figure 2. Monthly harvested above-ground biomass from January (1) through October (10), 2021. Top grey panel indicates cool-season forage treatment. Grey right-side panels indicate warm-season N fertilization (applied after the May 2021 harvest). Colors denote warm-season pasture (BG='Pensacola' bahiagrass, BGRP = 'Pensacola' bahiagrass+'Ecoturf' rhizoma peanut).
Fig
Figure 3. Root and rhizome biomass from soil ingrowth cores harvested in January (1), May (5), and September (9) 2021. Cool-season forages (colors) present in January 2021 while warm-season dominance (bahia, bahia+rhizoma) returned in May and September. (bahia='Pensacola' bahiagrass, bahia+rhizoma = 'Pensacola' bahiagrass+'Ecoturf' rhizoma peanut).

 

 

Participation Summary

Educational & Outreach Activities

1 Tours
1 Webinars / talks / presentations
1 Other educational activities: Undergraduate student volunteer Hannah Shareen presented a poster related to her work on this project at the fall 2021 University of Florida Center for Undergraduate Research Center symposium in Gainesville, Florida, USA.

Participation Summary:

20 Ag professionals participated
Education/outreach description:

To date, twelve University of Florida undergraduate students and a visiting scholar from Brazil have been involved in carrying out the project activities. Through their participation, they are gaining valuable field and laboratory research experience. A student intern will be involved over the summer to finish the final field and laboratory activities. Additionally, one student volunteer recently secured a University Scholar Award to collaborate on the soil microbial biomass C and N component of this project.

Project Outcomes

1 Grant received that built upon this project
2 New working collaborations
Knowledge Gained:

Key lessons learned about sustainable agriculture thus far:

  • Pasture evolution: Warm-season pastures were established in 2015 in plots of sole 'Pensacola' bahiagrass and mixed 'Pensacola' bahiagrass+'Ecoturf' Rhizoma peanut. Seven years after establishment, we observed that 'Ecoturf' rhizoma peanut spread into sole 'Pensacola' bahiagrass plots. This coincides with previous research on rhizoma peanut that reported successful colonization of C4 grass pastures over time (Mullenix et al., 2014).
  • Cool-season forage biomass. Guidelines from UF/IFAS suggest that grazing of cool-season forages begin once plant heights reach 20-25 cm. Limited cool-season forage growth impeded harvests until late-February 2021. This was later than anticipated. Despite irrigating cool-season forages for the first month after seeding, limited rainfall during December and January likely contributed to slow growth. A moist February provided rainfall that promoted sudden and rapid cool-season forage growth.
  • Establishment. Cool-season forages were seeded within the UF/IFAS recommended date range for the study location. Although the warm-season pasture was apparently no longer growing at this point in the season, and despite trimming the warm-season pasture to 5 cm before no-till drill-seeding cool-season forages, it is likely that warm-season pastures competed with newly-planted cool-season forages. Experimenting with alternative cool-season forage seeding date (e.g. after the first frost) and/or establishment method (e.g. applying an herbicide to kill warm-season foliage) may limit the competition of warm-season species to improve cool-season forage outcomes.

These findings motivate an adaptive approach to management of cool-season forages in the upcoming season. There is still much to be learned from this system.

Recommendations:

Year 2:

  • A complementary study was established in November 2021 at the UF|IFAS Plant Science Research and Education Unit in Citra, Florida, USA to build off of the observations in year 1 (see first bullet point below). Cool-season grasses and cool-season grasses+legumes were established under three land preparation strategies (tillage, no-tillage, no-tillage+herbicide) and four N fertilizer rates (0N, split 30-50lbsN, split 50-30lbsN, and 80N) treatments. Irrigation was applied to all plots for the first month after planting. Leaf area index (LAI; a measure of light interception) and above-ground biomass was harvested 60, 90, and 120 days after planting. Data from year 1 of this new study is being analyzed.

Year 1:

Several project ideas have emerged as a result of the observations made during the first season of this project. These include:

  • Timing and method of cool-season forages in warm-season pasture. Suggested treatments would be to 1) kill warm-season foliage with a contact herbicide prior to overseeding cool-season forages, 2) overseed cool-season forages after the first killing frost burns warm-season foliage. An additional layer could be study irrigated vs. non-irrigated cool-season forages.
  • Below-ground activity of warm-season pasture and cool-season forage establishment. Is warm-season root and rhizome activity competing with newly-established cool-season forages? For example, is N immobilization by warm-season pasture limiting cool-season forage acquisition during initial growth stages?

 

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.