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Rotational Grazing Systems

By Jennifer Mohler, Bridger Scientific, Inc.


May 2009 Issue  



Part 1

     Montana and most states in the Northern Rockies have arid landcapes, resulting in limited grass recourses for grazing animals, especially those with small acreages.

     Since 2003, I have helped landowners understand and manage their resources by promoting sustainable practices and conservation-minded weed and grazing management plans.

     For over eight years, I have managed two to four horses on my ten acres north of Belgrade , Montana , and I have gained a deeper insight as I “practice what I preach.”


     In this article, I want to share some concepts that have allowed me to balance my horses’ needs and my land’s limitations. To put it simply, if you don’t manage your horses’ time on pasture, you’re asking for weeds. Below, I will go into more detail, but first I need start with the big picture.

Why Manage Grazing?

·        Reduces soil erosion      

·        Improves water quality

·        Enhances wildlife habitat

·        Increases forage production

·        Increases grazing capacity

·        Improves use of forage supply throughout the year

·        Maximizes efficiency of
your time and resources

·        Allows for flexibility in management

·        Saves money


     In order to ensure your land and its resources remain healthy, you need to have a plan. For example, when horses are left on pastures 24/7, they quickly overgraze because of their close and uneven grazing patterns. Horses prefer sweet, young grass, and in confined pastures will literally eat down those “sweet” spots to the soil surface, often killing the grass and creating bare spots while more coarse forage goes untouched.

     Additionally, horses’ high-impact movements, especially those of shod horses, uproot tender plants and dig long furrows in the soil, leaving welcome mats for weeds.

     As a result, desirable forage will decrease, weeds will increase, and management costs will skyrocket.

     Most properties benefit from a rotational grazing system. Pastures are subdivided into smaller areas or paddocks, and a portion of the pasture is grazed, while the other paddocks rest. This allows the rested grasses to renew energy reserves, rebuild plant vigor, and compete against weeds, ultimately improving the sustainability of the pasture resources. A rotational grazing system mimics nature by allowing our animals to move to a new area.



     The foundation of a rotational grazing system is a drylot or sacrifice area. This area is “sacrificed” so the rest of your land can grows grass. These areas keep animals off pastures to avoid overgrazing, soil compaction, and damage during wet (or very dry) weather. The drylot is also used to keep horses off pastures when you are fertilizing or using herbicides. Take into consideration how many animals you have and what their management needs are when setting up a drylot.

     Dividing pastures into several smaller areas and rotating among them helps avoid overgrazing, because a good portion of your land’s grass is growing while one pasture is being utilized.

     Allowing your animals to graze grass down to four-inch stubble and then moving them to another section encourages plant growth, limits soil compaction, and discourages parasite infestation. Most parasites congregate between the soil surface and two inches up the forage plant. Removing animals before plants are grazed too low helps keep your herd healthy.


Grazing Systems Keep Horses & Land Healthy

In Rotational Grazing…

• Pastures are subdivided into smaller areas or paddocks

• A portion of the pasture is grazed, while the other paddocks rest

What type of animal?

     When planning for a grazing system, the first consideration is what type of animal you have.

     Each species of animal has its on characteristics, needs, and grazing habits. I’m going to use cows and horses to illustrate my points, but know that all animals have unique and individual impacts on the land.


     Cows are creatures of habit, and they notice and respond to any unusual change in their routine. They are heavy animals and can inflict damage to pastures, especially wet pastures.

     Cows graze eight hours and ruminate for twelve hours, and due to the design of their lips, teeth and jaw, they can’t get closer than two inches from the soil. They are cloven-footed animals, meaning their hoof consists of two digits.

     Cows are known to camp out in riparian zones, but behavior can be modified with herding and placement of water and minerals.


      Horses are large, active, playful animals. Horses can quickly overgraze a property and damage resources when not managed properly.

     Horses tend to spot graze, overgrazing some areas and ignoring others. They eat smaller amount per session, with more sessions per day than cows. Their top and bottom teeth allow them to graze down right to soil surface, destroying the growing points of grasses.

     Horses tend to section pasture into eating and spoiling areas, leaving a lot of quality forage untouched. Their close and uneven grazing patterns coupled with high-impact movements, especially those of shod horses, can quickly stress the grass resources.

The goal is to create a grazing system that complements your property. Above is a map of my property. You can see I have five pastures, a drylot with two  shelters, and a perimeter shelterbelt. It would be ideal to have all pastures the same size, but irrigation ditches and the location of the house made it difficult.


What is an Animal Unit Month (AUM)?

     An AUM represents the approximate amount of forage consumed by one 1,000 lb cow in one month.

     AUMs were developed to help land managers determine the stocking rate or the number of animals that can be effectively grazed on an area of land. The stocking rate will vary greatly depending both on the type of livestock, the fertility of the land, and the climatic conditions.

     Many agencies use AUMs to help landowners determine the carrying capacity of their property.






weight gain

weight management


graze 8 hours, ruminate 12 hours

highly selective grazers

graze smaller amount per session, with more sessions per day


heavy, slow animals that tend to camp in areas of good grazing

active animals with high impact hoof action


use a grazing system that promotes desirable grasses

use a grazing system that promotes desirable grasses

Feed (hay)

0.4 tons/month

0.5 tons/month

Forage (grazing)

1.0 AUMS

1.25 AUMS

Notice that the last row in the table shows that a cow represents 1 AUM, 
and a horse 1.25 AUM.


  Caution!! The AUM concept is based on a cow grazing 24 hours a day, not a horse. Recall that horses behave very differently in pastures than cows, and allowing 24 hour access to horses will have fundamentally different impacts on the land and horse health (more on that later).

  Even though it’s not perfect, calculating the carrying capacity of your land is the starting point of sustainable land management.


What can your land sustain?

     To help explain AUMs, I’ll use my place as an example. Hang in there with me through these calculations, because at the end of this I will discuss the limitations I find in the AUM method and explain the method I use that yields great results.

Question: How many horses can I graze on my 6.5 acres and for how long?


6.5 acres of pasture. Even though my property is 10 acres, I have only 6.5 acres of pasture.

     Since I have water rights, I am able to irrigate. I will calculate both dryland and irrigated numbers to illustrate the difference between them – it’s amazing!

     Soil AUMs: 0.5 for dryland and 3 for irrigated (note: numbers used are for ideal conditions)

     To find the AUMs for your property, contact your local NRCS office or look up soil reports on line at the Web Soil Survey (



Dryland Pasture

Irrigated Pasture

Soil Report AUMs

0.5 AUM

3.0 AUM

Number of Acres

6.5 acres

6.5 acres

Total AUMs for pasture

 = acres x AUMs

3.25 total AUMs

19.5 total AUMs

Total carrying capacity

= AUMs/Animal Unit Conversion Horses = 1.25, cows = 1

*same at their AUM requirements in table 1

2.6 Horses

15.6 Horses

Table 2: Calculating carrying capacity

(Source: Tips on Resource Conservation for Landowners Who Manage Smaller Tracts of Land in Montana , NRCS)


So what does this mean?

6.5 acres of dryland pasture can carry…

2.6 horses for 1 month, or

1 horse for 2.6 months, or

2 horses for 1.3 months

6.5 acres of irrigated pasture can carry…

15.6 horses for 1 month, or

1 horse for 15.6 months, or 2 horses 7.8 months


     What a difference water makes! According to the math, with irrigation I can easily manage 4 horses for 3.9 months - right? Not with 24/7 access! Remember, horses are highly selective when grazing and can quickly change the composition of species to favor weeds and unpalatable grasses. For two years, I had 4 horses and found I could graze for 6 months and still have healthy grasses at the end of the year.

To maintain a sustainable system, I follow two simple rules:

Limiting the time horses are grazing/out on pasture to 5 hours per day.

Stop grazing with 4 inches of stubble left.

Next month, Part 2 of the article — Limiting a horse’s time spent grazing and the four-inch of stubble rule.


Map © Jennifer Mohler 2003-2008

Jennifer Mohler of Bridger Scientific, Inc., is a resource conservationist specializing in holistic and sustainable land management. Jennifer has worked with a diverse group of public, private, and non-profit entities to conserve natural resources by promoting sustainable practices and conservation minded weed and grazing management plans. As a horse specialist, she assists landowners in finding a balance between the horse’s needs with the land’s limitations and offers creative options to reduce costs, increase forage yields, manage weed populations, and improve animal health. You can reach Jennifer Mohler at 406-388-5668 or


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