The Grass is Greener Underneath the Fence

Cattle grazing underneath a fenceline. The animals may know best where to find some of the healthiest grass on the farm.

When it comes to rotational grazing, our job is to keep providing that fresh, “greener grass” that’s on the other side of the fence to the animals—in a controlled way. Yet if we look at an old, undisturbed fenceline, we often find the greenest, tallest grass growing underneath the fence. This strip reveals what our pastures could be—if we prevent compaction and allow the soil to breathe.

Soil compaction often happens slowly under the weight of hooves and tires. But its effects can be profoundly negative. Studies suggest immediate crop production declines of 15%, including grass production. As heavy livestock, trucks and tractors compact soil over time, pore spaces that should hold air and water become compressed. Ideally, half of a soil's volume should be made up of these pore spaces. Without them, water can’t soak in, oxygen can’t circulate, and plant roots can’t grow.

A tractor’s tires may distribute weight more broadly in the topsoil than an animal’s hooves but its axle weight compacts the soil more deeply. Image credit: Fay Benson, previously of South Central NYDairy and Field Crops Team

Once compaction reaches 300 psi, roots effectively stop growing downward, spreading sideways instead. Shallow-rooted plants can’t reach water during dry spells, and will suffer under the heat and drought conditions of summer. And when grasses start to struggle, weeds that thrive in compacted conditions eagerly take their place.

It’s not just plants that suffer from soil compaction—the soil itself becomes less healthy.  Beneath our feet are networks of fungi, earthworms, macro-invertebrates, and billions of bacteria. Extraordinarily, their combined biomass in healthy soil can outweigh the livestock grazing above them. Many of these organisms live in the pore spaces that compaction destroys. Like animals above ground, they need air to survive. Without it, they die, and the soil loses its ability to decompose organic matter and recycle nutrients.

Compaction changes the soil’s physical structure, damaging the mix of large and small pore spaces that are part of good soil aggregate. Poor drainage leads to more ponding and potential for anaerobic conditions that can exacerbate denitrification. It also leads to more runoff, followed by increased erosion. When we unwittingly compact the soil, we suffocate the ecosystem that powers everything from forage growth to carbon cycling—all of which fuels the health of our pastures and herds.

Soil on the pasture surface is sodbound to a 3-inch depth, with severe compaction below, where it breaks into large horizontal clods and contains few roots. Photo credit: Justin Morris

Preventing compaction is far easier—and cheaper—than trying to fix it. Start by keeping soil covered with vegetation, increasing organic matter, and monitoring fertility to encourage healthy plants. Bare soil is most vulnerable. Deep-rooted grasses help build organic matter and soil structure, allowing the soil to act like a sponge, absorbing and retaining water. Healthy plant cover also contributes more biomass and more plant litter, cushioning traffic from animals and equipment.

Even simple practices can help. Before spreading manure from his annual bed pack, a local farmer first creates a thick base layer to form a 'manure road'—a path he’ll repeatedly use to access the fields with future manure loads. The thick layer helps distribute the weight of the tractor and spreader, reducing possible compaction. When he’s done spreading, he’ll harrow the manure “roadway.” Oftentimes, the best tool for soil health is simple awareness.

This is especially true when we move equipment or livestock over a field. Pastured livestock farmers already have good reasons to move cattle frequently, such as access to fresh grass, preventing overgrazing, and avoiding manure buildup--add minimizing compaction to that list. Longer grazing residencies are correlated with increased soil compaction. Cattle can actually exert more contact pressure on the soil than equipment, up to 29 pounds psi, leading to compaction primarily in the topsoil. While equipment may exert up to 21 pounds per square inch on the soil, the axle weight load can create lower subsoil compaction that may not be able to be rectified. While critical axle load will vary by soil types and moisture, plan to keep axle weight loads to absolutely no more than 10 tons per axle, preferably less than 6 tons per axle.

The same tire shows different stress patterns at varying pressures: low pressure concentrates impact  near the edges in a smaller area, while high pressure shifts stress beneath the center, covering a larger area. Excerpted from “Conservation Tillage Systems and Management” by Schuler, Casady and Raper. 

While up to three-quarters of compaction can occur on the first pass, longer or more frequent passes tend to produce deeper and more lasting compaction. If you’ll need frequent vehicular access to pastures, consider designating a permanent lane or roadway to contain the negative effects of compaction. And when using equipment in pastures, consider the vehicle’s overall weight. When possible, choose wider tires and/or lower tire pressure. Both will lower the pressure per square inch by increasing the contact area, distributing weight more evenly. The wider the footprint, the less damage and compaction to the soil.

Soil types also matter. Clays and bottomland soils are particularly vulnerable to compaction—especially when wet. Timing is critical. Avoid heavy grazing or machinery use after rain. Sometimes the physical properties of the soil can be altered to help prevent compaction. Perennial wet spots may be able to be drained. Tight, heavy soils can be amended with calcium to “loosen” them up. 

If your soils are already compacted, there are ways to help repair them, but none are flawless. Tillage or subsoiling can help break up compacted layers but should be used sparingly and always followed by the long-term management changes described above. Otherwise, you’ll end up right back where you started.

Sometimes, the best solution is to change how the field is used, at least in the near term. Graze less, cut for hay, or add compost and manure to increase organic matter to rebuild soil structure. Allowing time to work through various wet/dry and freeze/thaw cycles can help alleviate some topsoil compaction. Creating contingency plans and maintaining flexibility can make big differences with many of our pasture management strategies.

Managing compaction helps protect the life beneath our boots, the unseen web that ultimately supports every blade of grass and thus every animal that grazes that flourishing field. Next time you walk your pasture, pause at the old fenceline. Let it remind you what’s possible when we tread purposefully and manage with care.

Feel free to reach out with feedback or questions—we’re here to help!

Find more posts and grazing resources here: https://www.northjerseyrcd.org/grazing-resources

Click links below for other articles in the Flourishing Fields Series:

1st post: Understanding Your Soils

2nd post: Grazing Animals are Good for the Grass

3rd post: Timing is Everything

4th post: Making Sugar While the Sun Shines

5th post: The Disruptive Art of a Flourishing Field

6th post: This post

Craig Haney

Chaney@northjerseyrcd.org