Maximizing Forage Quality Through Targeted Grazing of Native Warm-Season Grasses

Warm-season grasses are generally thought to be of lower quality than cool-season grasses. However, tropical or subtropical climate regions have highly productive grazing systems based primarily or solely on warm-season grasses. This brings up the question—can targeted grazing management increase the quality of warm-season grasses in temperate regions?

A strong connection exists between forage quality, proportion of leaves, and animal performance, due to matching nutritional requirements and increased intake (Congio et al., 2018). To maximize animal growth through optimal forage quality, it is essential that they graze when the proportion of leaves is at its greatest. By setting grazing targets that maximize leaf proportion, native warm-season grasses (NWSG) can produce high-quality forage.

In forages, leaves are the most nutrient-dense component, providing richer protein concentrations, lesser fiber levels, and greater digestibility than stems or other structural components. In addition to their superior nutritional value, leaves play a critical role in supporting photosynthesis, promoting plant regrowth, boosting forage yield, and enhancing pasture persistence. Animals prefer to consume leaves due to their high nutritional quality, which also allows for greater intake and subsequent improvement in animal growth (Sollenberger & Burns, 2001).

During regrowth following a defoliation event, grasses primarily focus on producing leaves. As plant density increases, competition for sunlight intensifies, leading to stem elongation and a subsequent reduction in overall forage quality. Although stems contribute to forage mass, they are of low quality and are often avoided by animals. Research has shown that at a specific stage in the regrowth cycle, the leaf-to-stem ratio reaches an optimal point that provides the highest proportion of leaves (Da Silva et al., 2015). This occurs when the pasture intercepts 95% of incident light, allowing only 5% of
light to reach the ground (Figure 1, left picture).

Pasture Height for Optimal Leaf Production

A greenhouse experiment determined the pasture target heights that provide forages with the greatest proportion of leaves and nutritive value. Various warm- and cool-season grass species were grown individually, with light interception and canopy heights monitored every 2–3 days. Plants were clipped once the canopy intercepted 95% of light.

At each clipping event, 50% of the pre-cutting height was removed (Figure 2). Post-grazing residual is crucial for forage regrowth and long-term pasture persistence. Leaves play a key role in promoting faster regrowth of quality forage. Research suggested that removing 50% of the initial pre-grazing height is sufficient to provide animals with high-quality forage while also allowing for quicker regrowth and additional grazing cycles during the season (Martins et al., 2021). The removed forage was analyzed for nutritive value in the Sustainable Agroecosystems Lab at The Ohio State University. Measurements were taken from August 2022 through August 2023.

Results showed that forage target heights that maximize leaf production were 20 inches for the native warm-season grasses, Indiangrass and big bluestem; 13 inches for the cool-season grasses, tall fescue and orchardgrass; 12 inches for Kentucky bluegrass; and 8 inches for white clover (Table 1). The nutritive value of the warm-season species were similar to the cool-season species (Table 1) and comparable to premium-grass hay quality, according to USDA hay-quality guidelines.

Table 1. Forage target heights for maximizing leaf proportion and nutritive value in warm- and cool-season grass species. Forage was cut at the heights specified below, with 50% of the upper portion removed.

Forage Height for Greater Leaf Proportion in Forage Mixes

 A field experiment studied different forage mixes to evaluate if the averaged optimal heights from the greenhouse experiment applied to field conditions for a mix of cool- and warm-season grass species. Pre-grazing target heights were calculated by averaging the optimal heights for individual species to maximize leaf production in all species.

Three grazing strategies were evaluated (Table 2):

1. A typical Ohio tall fescue and white clover mix grazed at a higher target height (16 inches), and removal of 70% of available forage (Figure 1). This represents traditional grazing management focused on forage mass accumulation and intensive grazing.
2. A diverse cool-season mix including tall fescue, orchardgrass, Kentucky bluegrass, and white clover, grazed at a shorter target height (12 inches) to maximize leaf proportion and removal of 50% of available forage.
3. A native warm-season mix including big bluestem and Indiangrass, also grazed to maximize leaf proportion (20 inches) and removal of 50% of available forage.

Frequency x Forage Mass

Grazing cool-season mixes at a higher target height (16 in.) with a greater removal rate (70%) (Figure 2) resulted in more available forage before grazing mass (pre-grazing mass minus post-grazing mass) (Table 2). However, much of the mass consisted of dead forage material rather than green leaves, which is often related to lower forage quality.

This management strategy resulted in two grazing cycles from May to November during the 2023 growing season. In contrast, a management strategy grazing cool-season mixes at a shorter target height (12 inches) with less removal (50%) - a strategy designed to maximize leaf proportion and promote faster regrowth - produced less overall forage mass but significantly more green material. This result is usually associated with better forage quality (Table 2). This approach also allowed for four grazing cycles during the season. Similarly, grazing warm-season grasses at shorter heights (20 inches) with lesser removal rate (50%) resulted in a greater availability of green material and supported three grazing cycles in 2023 (Table 2).

Considerations for Fertilization on NWSG

Native warm-season grasses are adapted to low-fertility soils and produce large amounts of forage mass without fertilization. However, to improve nutritive value and promote faster regrowth, fertilization is recommended based on soil tests for phosphorus (P) and potassium (K), or nitrogen (N) for yield goals. Although no specific recommendations for NWSG are available, P and K are typically applied when soil levels are low (Keyser et al., 2012).

Over time, N recycling from manure can potentially reduce the demands of N fertilization if pasture is well-managed under rotational grazing, especially with more frequent grazing cycles (Smith et al., 2009). N recommendations for NWSG suggest that applying 60 lb of N per acre can boost forage production, with an additional 30 lb of N per acre recommended for achieving higher yields (Keyser et al., 2012). A multi-site study in Tennessee found that optimal N rates for big bluestem, switchgrass, and eastern gamagrass production ranged from 60 to 120 lb of N per acre, with higher rates (up to 180 lb of N per acre) enhancing crude protein level (Bisangwa et al., 2024).

During our study, fertilization rates were the same for cool-season and warm-season forages. Nitrogen fertilization was applied at a rate of 90 lb of N per acre, split in two applications (spring and fall for cool-season forages, and early and late summer for warm-season forages). This strategy supported above-average forage yields, faster regrowth, and more frequent grazing cycles. Lime was applied to a target pH level of 6.5, while P and K were applied to maintain levels above critical thresholds (LaBarge & Beers, 2022) to promote pasture longevity.

Preliminary results from greenhouse and field experiments suggest that balancing grazing heights and removal rates to maximize leaf proportion and promote regrowth can significantly enhance forage quality and grazing frequency, benefiting pasture health and animal growth. Using target heights for grazing management can be an easy and practical approach to optimize pasture defoliation. This approach is inexpensive and open to creativity, where any tool capable of measuring the pasture height can be viable.

Interesting examples are grazing sticks, painting the fences at the wanted targets, wrapping tape around boots to identify the target height, and others.

A good tip to check the accuracy of your average pasture height estimation is to observe the height where the pasture maintains its density. Figure 3 illustrates a good and a possible bad estimation of a pasture height.

Summary

Maximizing leaf proportion

Targeted grazing at shorter heights to optimize the leaf-to-stem ratio, in both cool- and warm-season mixes, has resulted in greater availability of leaves. This approach also improves forage production and supports more frequent grazing cycles for the whole grazing season.

Impact of grazing height and removal rates

Cool-season mixes grazed at a higher target height (16 inches) with greater removal rates (70%) resulted in more forage mass available to graze, but much of it consisted of dead or low-quality material. In contrast, grazing at lower heights (12 inches) with a 50% removal rate resulted in less available forage mass but a greater proportion of green leaves, improving annual forage production.

The warm-season mixture managed with shorter grazing height (20 inches) and moderate removal rates (50%) produced almost twice as much green forage and half as much dead material as the cool season mixtures. This illustrates how warm-season grasses can significantly respond to grazing management.

Post-grazing residuals

Leaving 50% of the available forage as post-grazing residual effectively maintains faster regrowth and long-term pasture persistence. This strategy allows for more frequent grazing without depleting the pasture's productivity.

Grazing frequency

More frequent grazing cycles, respecting plant regrowth needs, were achieved with lower grazing heights and reduced removal rates. Shorter, more controlled grazing promotes quicker regrowth and higher-quality forage (Table 1), making it a more sustainable strategy for pasture management.

References

Bisangwa E., Richwine J. D., Keyser P. D., Ashworth A. J., & Walker F. R. (2024). Native warm-season grass response to nitrogen fertilization. Agronomy, 14(1), 180.
doi: 10.3390/agronomy14010180

Congio, G. F. S., Batalha, C. D. A, Chiavegato, M. B., Berndt, A., Oliveira, P. P. A., Frighetto, R. T. S., Maxwell, T, M. R., Gregorini, P., & Da Silva, S.C. (2018). Strategic grazing management towards sustainable intensification at tropical pasture-based dairy systems. Science of the total environment 636(15), 872–880.
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Da Silva, S. C., Sbrissia, A. F., & Pereira, L. E. T. (2015). Ecophysiology of C4 forage grasses—Understanding plant growth for optimizing their use and management. Agriculture 5(3), 598–625.
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Keyser P., Bates G., Waller J., Harper C., & Doxon E. (2012). Grazing native warm-season grasses in the Mid-South, native warm-season grasses [PDF]. University of Tennessee, Center for Native Grasslands Management.
utia.tennessee.edu/publications/wp-content/uploads/sites/269/2023/10/SP731-C.pdf

LaBarge, G., & Beers, L. (2022). Nutrient management of forage crops intended for hay [Fact sheet]. Ohioline. Ohio State University Extension.
ohioline.osu.edu/factsheet/anr-0109

Martins, C. D. M., Schmitt, D., Duchini, P. G., Miqueloto, T., & Sbrissia, A. F. (2021). Defoliation intensity and leaf area index recovery in defoliated swards: Implications for forage accumulation. Scientia Agricola 78(2), 1–8.
doi:10.1590/1678-992X-2019-0095

Smith, S. R., Lacefield, G., & Keene, T. (2009). Native warm-season perennial grasses for forage in Kentucky [Fact sheet]. Extension Publications, University of Kentucky, College of Agriculture.
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Sollenberger, L. E., & Burns, J. C. (2001). Canopy characteristics, ingestive behavior and herbage intake in cultivated tropical grasslands. International Grassland Congress.
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This work is supported by AFRI Sustainable Agricultural Systems Coordinated Agricultural Project (SAS-CAP) grant no. 2021-68012-35917 from the USDA National Institute of Food and Agriculture.