In the northern United States, hayfields often consist of a single forage species like alfalfa, but single species pastures are somewhat rare. Most single species pastures will be annuals like sorghum-sudangrass that are used to fill temporary forage needs. Otherwise, permanent pastures are almost always seeded as a mixture of species. There are many good reasons for this.
The classic example is mixing a forage grass with a legume like red or white clover, birdsfoot trefoil, or alfalfa. Through symbiosis with beneficial rhizobia bacteria in root nodules, legumes are able to fix all the nitrogen (N) they need from the N present in air. Nitrogen is then recycled in soil when legume roots and shoots die, and about half of the N in plant protein that is consumed by grazing livestock is returned to the pasture as manure and urine. As a result, legume N cycles within a well-managed pasture and ultimately feeds the grasses.
Nitrogen from legume and manure sources is released over time because it is gradually decomposed into available N rather than providing a pulse dose that is instantly available the way unstable fertilizer N is. This helps even out the availability of forage across the growing season because N is a primary driver for plant growth. It also helps reduce loss of N from the soil, which can happen whenever mineralized N is present in amounts greater than what the pasture plants need. Holding onto the N in the pasture saves producers money and helps prevent contamination of groundwater and surface water.
The rule of thumb is that a pasture containing at least 30% legume by weight of forage will provide all the N needed for adequate plant growth. While pasture growth may be further improved by adding N fertilizer, producers should take a hard look at whether that is cost-effective in their specific situation, especially when N fertilizer prices are high.
Adapt to differences
The second big reason for adding biodiversity to pastures relates to adaptation of species to microsites within a field. Pastures rarely present a consistent set of growing conditions across the entire area. Soil types differ from sandy to heavy. The degree of slope and drainage potential varies. A north-facing slope will have different sunlight availability, temperature fluctuations across the season, and potential for winterkill than a south-facing slope. Soil fertility is certain to vary across a single pasture and can be greatly influenced by past cropping history and grazing management. While it is difficult to find a single forage species that works well across the board for all microsite combinations, graziers in Northern regions are fortunate to have a wide variety of species that grow well with each other and are well-adapted to these different conditions.
Most seed marketers sell pasture mixes designed to cover the bases for the typical microsites found in their service region. Many will also make custom mixes according to producer specifications. It is important to recognize that these mixes are not intended to create a pasture that has exactly the same proportion of species on every square foot. The species in the mix will segregate to the microsites where they are best adapted and may not establish or persist at all on microsites where they are unsuited. This is normal.
Expensive seeding mistakes can be avoided with a little knowledge about your site. For example, if the site is sandy and prone to drought, it is pointless to buy a mixture that mostly contains species with poor drought tolerance. Likewise, if your pasture has questionable drainage for part of the year, don’t buy mixtures with a high proportion of species that hate wet feet. Good species for drought-prone sites or sandy soils include alfalfa, orchardgrass, smooth bromegrass, meadow bromegrass, and chicory. Heavier soils with marginal or seasonally wet drainage favor red and white clover, birdsfoot trefoil, Kentucky bluegrass, timothy, ryegrasses, tall fescue, meadow fescue, and festulolium.
Don’t hesitate to make use of species that volunteer, or seed themselves, into a pasture. As long as a pasture “weed” is not toxic and livestock will eat it, it is feed. Quackgrass, dandelion, plantain, and many others add diversity and are readily grazed and nutritious.
Consider growth curves
Lastly, pay attention to the compatibility of forage species. Choose species from different functional groups — cool-season grasses, legumes, and nonlegume forbs — where normal peak growth happens at staggered times of the year. This will help reduce competition for resources and ensure a more consistent forage supply over the growing season. Orchardgrass and Kentucky bluegrass tend to have early spring growth peaks, followed by fescues, then bromegrasses, with timothy and perennial ryegrass peaking late. Legumes also have the greatest growth in spring but tend to have a more even growth pattern than grasses over the rest of the growing season.
Producers often wonder about growing warm-season native grasses like bluestems, indiangrass, switchgrass, or prairie plants in mixes with improved cool-season grasses and legumes. Unfortunately, this rarely works well in Northern regions. The native warm-season plants have a relatively short growth window and slow regrowth after grazing, and they usually get crowded out by the fast-growing cool-season forages that have a much longer growing season. Warm-season perennial mixes in the North are best established as summer pastures dedicated to these species only.
There are several methods for enhancing forage species diversity in an existing pasture. Frost seeding, or broadcasting seed onto exposed soil in early spring, is an excellent method on heavier soils because freeze-thaw cycles bury the seed. Frost seeding is most successful with red or white clovers and birdsfoot trefoil, and it is an excellent way to add legumes to a pasture. It can also be used to seed some grasses, such as ryegrass and festulolium, although this doesn’t work as consistently as with legumes. I have also had frost seeding success with orchardgrass and chicory.
Almost any desired species can be no-till drilled into pastures. The best timing in the North is right before the pasture breaks winter dormancy. Overseeding during the growing season is risky because new seedlings may be overwhelmed by the rapid regrowth of established plants. This is one instance where some judiciously planned overgrazing right before seeding can be helpful to suppress regrowth of existing plants long enough for seedlings to get started. Avoid overseeding in the middle of summer because seedlings will have little chance to establish when water is scarce.
Another low-tech option for adding species to pastures is bale grazing; however, feeding mature hay will add seeds to your pasture whether you want them or not. Make sure you are planting desirable forage seeds from your hay, not weed seeds like thistles.
Regardless of method used, building forage species diversity will pay off with better pastures that hold up under stress and grow more forage for your livestock.
This article appeared in the February 2024 issue of Hay & Forage Grower on pages 16-17.
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