Project Lead/Researchers: Shelby Gruss, assistant professor, Department of Agronomy, Iowa State University; and Randie Culbertson, assistant professor, Department of Animal Science, Iowa State University
Overview
This project complements a USDA-FSA-funded study exploring the potential for cattle integration into Conservation Reserve Program (CRP) managed acres in Iowa, specifically evaluating forage nutritive value, species composition and biomass yields prior, within and following the primary nesting season. Typically, CRP is not valued for forage because grazing is only allowed before May 15 or after August 1. Given the fluctuations in forage quality resulting from plant maturity, species composition, and environmental conditions, this research aims to provide critical guidance to USDA-FSA, farmers, and conservationists, to inform the potential for sustainable grazing on CRP acres and similar natural areas, while still considering conservation goals.
Materials and Methods
Biomass samples were collected from five CRP fields that contained warm-season perennial grass stands during summer 2024 (Conservation Practice 25, 42, and 38). Fields were located throughout the state in five landform regions: Northwest Iowa Plains, Des Moines Lobe, Paleozoic Plateau, Southern Iowa Drift Plain and Loess Hills. Data collected included species composition (native warm-season grasses, cool-season grasses, legumes, forbs, bare ground, and litter), forage biomass and nutritive value.
Fields were sampled three times throughout the grazing season before, during and after the primary nesting season, from 10 locations. At each location sampling occurred above and below 6 inches (NRCS recommended grazing height for warm season perennials).
Results and Discussion
Cuttings throughout the analysis align with prior (May), during (June), and after (August) the primary nesting season, cutting 1, 2 and 3, respectively.
Variables for each analysis are described in the list below.
- Biomass
- ILR1 – Warm season compared to cool-seasons, legume, forbs and litter
- ILR2 – Cool-seasons, compared to legume, forbs and litter
- ILR3 – Legumes compared to forbs and litter
- ILR4 – Forbs compared to litter
- Nutritive value
- ILR1 - Warm-season compared to legumes and forbs
- ILR2 - Legumes compared to forbs
Compositional Data Shifts
Cutting timing influenced plant community composition. Warm-season grasses had the greatest shift in species composition, increasing throughout each cutting.
Other slight shifts were also observed: Legumes increased with later cuttings, while cool-season grasses, forbs and litter declined. Most of the changes occurred between the first and second cutting, with composition stabilizing thereafter. This trend aligns with seasonal temperature increases that favor warm-season growth and reduce the growth of cool-season species. This shift indicates that grazing too early could limit the warm-season growth.
Biomass
Biomass was compared as: above 6 inches, below 6 inches and total above biomass (tons acre-1 ).
- Biomass above 6 inches and total above-ground biomass was primarily affected by cutting, with each cutting increasing in biomass production. While biomass below six inches was mainly affected by ILR1 by cutting interaction (P< 0.05) and ILR3 (P<0.1),
These patterns may reflect growth forms and patterns of many of our native warm-season species. Many native warm-season grasses are tall bunch grasses, forming dense bases that grow tall and can thin out towards the top. Additionally, growth is slow early in the season (cutting 1), but it picks up exponentially with the increase in temperatures for cuttings 2 and 3.
Nutritive Value
Crude protein (CP) and total digestible nutrients (TDN) declined by the third cutting (August) (P < 0.05), with CP dropping by ~3% (Figure 4). Both CP and TDN were negatively associated with ILR1, indicating that higher warm-season dominance was associated with reduced CP.
Fiber components (neutral detergent fiber (NDF) and acid detergent fiber (ADF) increased with ILR1 and ILR2, and ADF showed a cutting by ILR2 interaction (P < 0.1). These patterns align with previous work that as forages increase in maturity, we see an increase in biomass and decline in nutritive value, specifically a decline in CP and TDN, with an increase in their fiber fractions.
Additionally, warm-season perennials typically have a higher fiber fraction compared to cool-season, forb, and legumes, as seen here. Interestingly, a high legume dominance compared to forb composition demonstrated a complex relationship by ADF percent by cutting timeframes. This could vary depending on annual forb vs perennial forb dominance, but this is speculative.
Conclusions
Integrating grazing into CRP management requires balancing forage availability and nutritive value. Early-season grazing offers higher quality but lower biomass, while late-season grazing maximizes quantity at the expense of quality. Warm-season dominance drives many of these trends.
This study was conducted on ungrazed CRP land, demonstrating the consistent decline in quality and increase in biomass. Incorporating targeted adaptive grazing strategies, potentially rotational stocking or grazing in specific times of the season, could balance forage quality and quantity while maintaining conservation goals.
Key Takeaways
- Species composition shifts: Warm-season grasses dominate later in the season, while cool-season grasses and litter declined.
- Biomass increases over time: Forage quantity is highest after the nesting season (August).
- Nutritive value declines as plants mature: Crude protein and TDN decrease from early to late season, while fiber fractions (ADF, NDF) increase.
- Warm-season species reduced forage quality: Warm-season dominance increased fiber fractions and declined CP and TDN.
- Trade-offs: Early grazing offers better quality but less quantity and potentially could negatively impact conservation purposes of CRP, especially for nesting birds. Late grazing maximizes biomass but reduces forage quality. Adaptive strategies like rotational or partial-season grazing for targeted conservation purposes could balance multiple goals.