This article accompanies a Cornell Equine Seminar presented April 14, 2020 by Jodi Letham, M.S. ‘19, extension associate with Cornell University Cooperative Extension.

The quality and health of your pasture has a significant impact on your horse’s health. Field crops specialist Jodi Letham, M.S. ’19, explains how best to manage your pasture’s ecological system to keep costs low and your animals thriving.

What follows here is an overview. Please watch Letham’s entire presentation for more extensive information.

Introduction to pasture ecology

Pasture ecology describes the interrelationships of sunlight, grasses, legumes, weeds, soils and grazing animals with their environment.

Categories to consider in pasture ecology include:

• non-living components
• plants that capture solar energy
• herbivore/carnivore consumers
• decomposers

All plants, animals, bacteria, protozoa and fungi occupy specific niches or places in the ecosystem. As some plants are better able to adapt to their environment than others, ecosystems have a huge impact on pasture quality. By understanding these systems, you can be a better pasture manager, who knows how to adapt to changes in weather and interpret how research and experience from other areas apply to your farm. 

What influences pasture ecology?

Many factors influence pasture ecology, including:

• energy
• water and temperature
• pasture site
• space available
• competition
• grazing habits
• plant anatomy
• soil characteristics
• decomposers, pests

Energy

“Free” energy from sunlight is the single most important influence on pasture yield.

“We want to manage pastures to maximize sunlight by increasing the number of leaves, the size of leaves and the length of the growing season,” Letham explains. Light collection is influenced by the forage species, leaf angle, canopy density, leaf aging and the availability of water and nutrients in the soil.

During the day, photosynthesis allows plants to capture energy from light. They fix carbon dioxide and water to make carbohydrates, some of which are converted into other plant components. Legumes such as clovers and alfalfa also use rhizobia bacteria and root nodules to fix nitrogen from the air and make proteins. Mineral nutrients from the soil are converted into enzymes, coenzymes, DNA, RNA and cell structures within the plant — and later within livestock that eat and digest the plants. 

At night, photosynthesis rests while respiration continues. In this process, the plant converts carbohydrates to carbon dioxide and water to perform its metabolic functions. As a result, the sugar content in plant leaves is at its lowest level at sunrise and increases over the course of the day. This may influence when you decide to harvest hay, depending on the amount of carbohydrates you hope to feed to your horses.

Sunlight

Unlike soil nutrients, sunlight is a resource that needs to be used instantaneously as it becomes available. Competition for sunlight is greatest in early growth in the spring and in regrowth after grazing and cutting hay. Shade from a neighbor can lower the photosynthesis rate and thereby the growth of a plant, so various species gain a competitive advantage from having different leaf arrangements that enable them to maximize the amount of sunlight captured.

Water

Rainfall is the primary water source for pastures. Irregular rainfall can lead to deficiencies or excessive soil water, such as runoff and soil erosion, and have a direct effect on plant productivity and persistence.

Drought, on the one hand, can decrease the plant’s ability to take up water-soluble nutrients. Plants may produce smaller leaves to prevent loss of energy and grow deeper roots as they seek deeper water sources. Drought also limits legumes’ ability to fix nitrogen and carbohydrate storage supply decreases. Nutrient levels are typically highest at the soil surface under these conditions.

In saturated conditions, on the other hand, water fills soil pockets, excluding oxygen needed by plant roots, and fungal root disease organisms will thrive and damage roots. Plan for a longer recovery period after wet conditions before resuming grazing. 

Impacts of climate change

Temperature increases due to climate change have both positive and negative impacts on forages, for example improving water efficiency but also decreasing forage quality. Warmer temperatures will affect the seasonal patterns of the types of grasses and species used in forages for livestock.

Pasture site

Pasture site is an important factor in planting forage, as crops cannot escape where they are located and the location affects the pasture composition and growth. 

Important factors to consider include:

• elevation
• soil type
• drainage
• slope and exposure

Rotational grazing

“I really like to promote rotational grazing if you can do it,” Letham says. Livestock is moved seasonally through paddocks that have been planted with species that mature at different times. “That way you can have optimal productivity in each pasture,” she explains. 

If rotational grazing is not an option, other practices are helpful, such as limiting the amount of time livestock spends in any pasture to avoid overgrazing and the growth of unwanted species.

Competition

All organisms grow and reproduce best when they live in an environment that provides ideal levels of their required resources, such as temperature, water, nutrients, solar radiation and protection from parasites and predators. If the availability of a resource falls below a critical value, the health and productivity of an organism decreases.

For example, clover needs a pH of 6.0 and high levels of phosphorus and potassium in the soil to thrive. At a lower pH, it may be outcompeted by tall fescue, which is more tolerant of such conditions. “As the stand ages, you’ll see some more tolerant species outlast others,” Letham says.

Competition between plants centers around such factors as water, nutrients, light, carbon dioxide, oxygen, environmental stressors and means of pollination and seed dispersal. “Increase the competitiveness of the desired species and or decrease the competitiveness of the undesirable species, given what you can manage,” Letham advises.

When planning inputs such as moisture and fertilizer, keep in mind the law of diminishing returns. They increase productivity to a point, at which the “bang for your buck” decreases and inputs can even case harm and negative returns.

Grazing habits

The intensity and frequency of grazing can have a tremendous effect on the competitive ability of pasture plants, which vary greatly in their ability to tolerate grazing. Factors that influence the productivity of a pasture include the number and species of animals (complexity), selective grazing, manure and urine, and traffic (how they animals move within the space). 

“If you’re overstocked on a pasture, the traffic is going to beat it up, so you’re going to have to micromanage more,” Letham says.

Pasture management

Horses that have unlimited access to pasture spend about 60 to 70 percent of their time grazing. As pattern grazers and selective eaters, they overgraze the tastiest grass first, which encourages bare patches and weedy areas to appear in heavily grazed pastures.

You can manage plant height, especially during leaf development and elongation, through grazing frequency, grazing intensity and soil characteristics such as 

• pH: Start with a soil test to determine lime and fertilizer needs, either by sending a sample to a lab or paying a service to do so. Fields being seeded to alfalfa, for example, need to have a pH of 6.5 to 7.0. Lime should be applied six months or more in advance to allow it to react with the soil and minimize herbicide carryover. Talk to your cooperative extension office to figure out which starter fertilizer package is right for your soil.
• fertility: Depending on their nutrient levels, various soils favor different plants.
• microorganisms: Soil organisms help reduce nitrogen loss through leaching and release organic materials for better uptake of nutrients. Rhizobium bacteria, for example, nodulate legume roots in a symbiotic relationship but require certain nutrients and pH levels for nodulation fixation of nitrogen to occur.
• earthworms: Earthworms can improve pasture production by 25 percent by aerating and loosening the soil, incorporating dead pasture plants, and eating nematodes that can harm clover roots. Note that using urea fertilizer can cut earthworm numbers in half, so use ammonium nitrate instead, and provide cover for them instead of bare soil.
• nematodes: Beneficial nematodes feed on bacteria, fungi and soil protozoa and promote rapid decay and incorporation of organic matter into the soil. 

Clovers

Among desirable plants for a horse pasture — though in small amounts only, for pasture species diversification — clovers have the advantage of being tolerant of somewhat poorly drained and acidic soils. Like alfalfa, however, they can cause bloat in animals, and they are generally short-lived and less productive than alfalfa. 

Clovers are planted in April to early May, usually mixed with grasses on pastures where the livestock will eventually be transitioned to pure grass. If oats are used, reduce the seeding rate by 30 percent.

Red clover is relatively easy to establish and grows on soils too wet or acidic for alfalfa, while white clovers are well-suited for pastures and do well in rainy years but are not recommended for silage because of their leafy growth habit.

Alsike clover – very similar to white clover, which has a white ring on its leaves – may cause issues in horses. Signs include excessive drooling, photosensitivity and big liver syndrome (liver toxicity). 

Weed management

Weeds are plants that grow where they are unwanted. They often fill in bald spots due to overgrazing. Several of these plants can be toxic to horses, including – in the Northeast – Alsike clover, Johnsongrass, burdock, jimsonweed, clocklebur and poison hemlock. “So you do want to watch and be careful,” Letham advises.

Weeds – from common, to invasive or noxious species – can cause significant economic losses due to their competitive and invasive characteristics. They may be grasses and grass-like plants, forbs (flowering broadleaf plants), legumes, trees, vines or shrubs as well as annuals, biennials or perennials. They may be spread by birds and other animals, wind and water, and by humans, both intentionally and unintentionally.

In competition with desirable plants, weeds take advantage of any opportunities that present themselves in the form of sunlight conditions, soil disturbance, moisture changes and soil changes. For example, their leaf shape may allow them to better capture light, and they may be able to withstand harsher conditions.

Weed control strategies

Good pasture management and mechanical measures such as mowing and brush hogging routinely can prevent the need for more expensive chemical techniques such as herbicides.

Herbicide application depends on the age of the weed as well as growing and weather conditions. It primarily targets broad leaves in a pasture. Broad-spectrum herbicide application is not common.

"I can’t stress enough that you’re definitely going to want to read the label so you get the right rate, make sure it’s registered for that use in your state and that you also understand the re-entry grazing interval when you can put the animals back,” Letham says. “Some windows can certainly be quite large.”

Another option for biological weed control is to use alternative livestock such as goats or host specific insects or diseases. You can use targeted grazing to achieve your management goals, for example by letting animals graze the pasture for a limited time so that it is not overgrazed before winter and plants can accumulate storage reserves during the cold season. 

Cool-season grasses

The Northeast is suited primarily for cool-season grasses, while the mid-Atlantic is a transitional zone down to the warm-season grasses of the South.

Perennial, tall-growing, cool-season grasses play an important role in New York agriculture as feed for livestock – grown alone or in mixtures with legumes – and for soil and water conservation.

Some main varieties are:

• Orchardgrass: high-yielding, good summer growth under intense cutting management, persists well with alfalfa; early maturing varieties are suitable for pure grass hay or silage, or with legumes for pasture; don’t use in mixtures with legumes for hay or silage; medium-late maturing varieties do well when combined with legumes for hay, silage or pasture
• Bromegrass: leafy, late-maturing, sod-forming grass that spreads by underground rhizomes and is rapidly propagated by seed; silage, hay or pasture – alone or in mixture with alfalfa, ladino clover or red clover; does not have rhizomes and producing flower heads in every regrowth, especially in fall; herbage and immature seed heads are palatable for livestock; Matua, the only variety available in the U.S., persists best if managed like alfalfa
• Timothy: hay-type forage grass; easily weakened by frequent cutting or grazing; decent for mixtures
• Reed Canarygrass: cool-season forage grass; does best in moist cool climates; prefers fertile moist and swampy soils, well suited for areas along waterways that frequently overflow; 
• Tall fescue: deep-rooted, long-lived, sod-forming grass that spreads by short underground stems; low-quality tall fescue forage – which can result in poor animal performance – is associated with the presence of a fungus (endophyte) growing in the plant; it is suggested that new seedings be made with endophyte-free varieties, and farms with pregnant mares should avoid this variety because the endophyte can cause abortions
• Perennial ryegrass: varieties can differ in their maturity by over two weeks; highly palatable bunch grass with no creeping growth habit; normally considered to be less winter hardy and slightly more digestible than other forage grasses, but newer varieties appear to survive satisfactorily, especially when grown in mixture with legumes
• Festulolium: cross between fescue and perennial ryegrass; avoid with pregnant mares

Watch Letham’s full talk online, and see the entire Cornell Equine Seminar Series videos here.