RONKS, Pa. — You may have thought that planting a cover crop and leaving it alone until termination and planting of the cash crop is the best way to get all the soil benefits of the ground cover. Cover crops by themselves certainly improve soil biodiversity, soil organic matter levels (which influence tilth and moisture capacity), nutrient cycling, and weed suppression, among many other benefits. And usually less disturbance means soil life and structure has the chance to flourish.
From a strictly soil health perspective, planting followed by mechanical harvest does defeat many of the soil improvement objectives of cover crops. Bringing animals out to graze the cover crop, however, may deliver even more soil health returns than a hands-off approach.
Think of rich, deep prairie soils. What made them that way? A combination of the impacts of grazing herds of buffalo and extreme biodiversity. It’s not unusual for over 100 plant species to be evident in any given area of a prairie! Obviously, you can’t create a synthetic version of this system on your finite acreage, but it can’t hurt to come a little closer to the ideal.
When grazing a cover crop, you close the nutrient cycle — similar to what you do when you plow a mature cover crop into the ground as green manure or spray and leave the residue to decompose. If managed right, grazing may actually be expediting this nutrient loop, since the deposition of manure and urine recharges soil organic matter and nutrients, arguably in a more plant-available form.
As they graze, animals trample organic matter into the ground, jump-starting the degradation process. Soil microbes are fed more easily, and they take it from there in integrating the trampled plant material and its nutrients into the ground. As soil organic matter increases, so does moisture-holding capacity. Its structure and tilth also improve – helping prevent compaction in the long term.
A common view of grazing has been that it contributes to compaction, which can become a reality if you graze when soils are too wet (especially if you already have heavy soils). Too much concentrated weight on wet soil weakens soil aggregates. As the cow walks, you have at least 300-500 lbs. concentrated over a few square centimeters of soil under the hoof that’s bearing weight. To maximize soil health benefits and minimize disturbance, make sure you are grazing half and leaving half. This gives hoofs more organic matter to trample in, while maintaining more residue protection for the soil ecosystem.
To manage that, it’s ideal to stock at high densities and move cattle at least once or twice a day. Frequently moving the animals can avoid the potential for compaction that arises when there is excess walking – when cows travel the same path repeatedly to get to water or supplemental feed, for example. They also tend to hang out more near fence rows and streams.
In the ideal scenario, no-till or minimum-till practices are used, and the combination of root biomass, root exudates and the animal trampling and manure improves soil physical properties. This can comes as a surprise to some who watch it in action. John Stigge, a no-tiller in Kansas, wrote recently in No-till Farmer:
The first year we grazed a cover-crop mixture of triticale and radishes that I seeded in wheat stubble. We grazed weaned calves there in late fall through early winter. They stayed on the field until it got too cold to haul water. The next spring our hired man, Luke, was planting corn in the field. We hadn’t grazed the entire field, keeping the calves on roughly half the acres. I was on the other side of the field and was surprised to hear the tractor pull down and struggle as it transitioned from the acres we grazed to the acres that hadn’t been grazed. That was the complete opposite of what I expected and it repeated over and over again the whole length of the field.
In theory, the ground we grazed should have been a little more dense and difficult to pull through due to compaction from hoof traffic, while the acres with cover crops only should have been nice and mellow. (See more at https://www.no-tillfarmer.com/articles/5708-deu#sthash.iDddFU9q.dpuf)
Annuals and perennials are both beneficial, but there’s a time and place for each. Short term perennial covers may be most beneficial for soil building, enhancing soil biology and carbon sequestration, but it’s really about what is the best thing to break up and vary the rotation. In a row crop field, a perennial of 1-2 full years may be the best break crop. Long-term root growth and animal activity helps build soil, and annual weeds and many pests get wiped out without the environment or host they are used to.
Cool season and warm season annuals also present an ideal opportunity to mix up the rotation, use a break crop while keeping a living cover growing year-round, and provide excellent quality forage in a very condensed time frame. This can be as simple as planting a triticale-crimson clover mix after corn silage comes off, and possibly getting some late fall and/or early spring grazing.
Give permanent pastures a rest. Extend the grazing season or simply take advantage of the late fall or early spring growth of winter small grains, legumes and brassicas. Or graze a diverse soil-building mix like Ray’s Crazy Mix when the summer slump sets in and pastures need longer rest periods. At the height of summer, you also want to be able to protect a field with a quick canopy of voluminous tall, viney growth. Even when you graze the summer annual, leaving at least half can let the residue continue to shelter the soil from extreme temperatures and hold onto moisture.
This can also be a great way to renovate a pasture for a season or more – and still let the animals take advantage of the forage.
Great gains are possible. If grazing timing is managed correctly, many cover crops make high quality feed, high in protein and digestible fiber. This can in turn boost milk production and daily gains.
Diversity builds the field and the rotation. Although a seemingly worn concept, diversity is always key in a successful rotation — in both the agronomic and nutritional sense. Each rooting pattern in a diverse mix delivers a unique contribution to soil building and feeding soil life. And each has a specialty when it comes to nutrient scavenging. Brassicas release an acid that helps pull calcium molecules away from other soil particles, while releasing phosphorus and sulfur molecules that are tied up in the soil. Daikon radishes and other deep rooted crops can break through hardpans and mine moisture and nutrients from the subsoil.
They are also often used to prevent leaching after a heavy nitrogen feeding. Legume roots host rhizobium bacteria that fix atmospheric nitrogen. Grasses have a dense root mass that can grow deep, scavenge nitrogen, and build soil organic matter. Many small grains and other species also have some degree of allelopathy, inhibiting the germination of other species (i.e. weeds). As feeds, brassicas and legumes contribute protein, while grasses boost digestible fiber and help slow the rate of passage of the richer components.
And more vigorous varieties can take off and get ground covered while slower-growing species in the mix catch up. The same goes for drier or wetter times – species that do best in those conditions can cover for those that don’t. A mixture sets up the ground and the animals for the widest range of benefits — never a bad thing.
Field operations are simplified. You can temporarily slash the labor and cost associated with harvesting and storing feed, as well as hauling and spreading manure (cows apply it for you).
Better weed suppression. Break up your current cropping system, and you disrupt weed lifecycles. Plus, when you regularly graze a field at high stocking densities, almost everything gets eaten. Animals don’t discriminate as readily.
A grazed cover crop is an excellent pasture renovation tool. Take out your permanent pasture and use a season or a full cycle of annuals to renew the field. And you won’t lose out on forage in the process!
It works if it leaves you enough time to grow ample biomass. You need to be able to grow enough material that the crop accumulates significant root mass and can be grazed while still providing ground cover and building soil with what remains. Wait to graze until there is adequate feeding potential – for some grasses this is eight inches, while many summer annuals can’t be grazed until they have 18 inches or more of growth.
Refrain from grazing on highly erodible land and close to water sources, as well.
Conserve water. Cover crops buffer the soil from the hot, dry extremes of summer. Very deep-rooted species like Daikon radishes and annual and Italian ryegrass can also mine moisture from below the plow pan.
In drier regions, blends almost always outshine single species. Species in a mix tend to help each other and work synergistically rather than competing. We can leave our weed mentality at the pasture gate – the idea that one species might become a threat to the others (of course, moderate seeding rates must be used for the seed sizes of each species). Diversity almost always leads to greater stability and survival.
Cover cropping to boost organic matter and overall soil health is relatively new to mainstream thinking. Introducing grazing takes these soil health impacts yet another step further. Each step of this has to be done strategically – you can’t approach cover-cropping as simply adding another cash crop to the rotation and expect the same soil health impacts of a cover crop. Sure, a year-round living root is highly beneficial, but so is keeping all the nutrients you grow in a closed loop. If the goal is truly soil improvement, make sure you’re growing a cover crop first and a forage second. Grazing fulfills the cover crop purpose if you can manage it to do so. As a result, you can look forward to improved soil resilience, nutrient cycling, reduced need for chemical fertilizers, and likely reduced weed pressure. (But look for this in the long term since soil conditions fluctuate from season to season and year to year.)
— King’s AgriSeeds Inc.