When designing a livestock grazing system to optimize animal health, pasture integrity, and economic productivity, the primary dynamic that must be accounted for is “grazing pressure”. Grazing pressure is the closeness to which pasture is grazed, i.e. the amount of grass/forb that is depleted from a pasture in a single grazing event. If the grazing pressure is too high, all three priorities suffer: animal health declines due to the depletion of desirable forage; pasture flora profile and soil structure degrade from overgrazing; and animal productivity decreases due to decreased nutrition. If the grazing pressure is too low, animal productivity (output) will increase because livestock eat as much as they want, but net profit suffers because forage (input) is wasted due to selective grazing. Optimum grazing pressure is achieved when livestock’s dietary needs are met and the least amount of forage possible is wasted.
Types of Livestock Grazing Systems
The dynamics of grazing pressure will be affected primarily by the “grazing system” used to manage a pasture. There are two basic types of livestock grazing systems: Continuous and Rotational. The following section on grazing systems is based partly on Penn State’s Agronomy Guide.
Continuous Grazing is the practice of grazing animals continuously on a single pasture for the entire growing season. The primary factor determining grazing pressure in this case is “stocking rate”, or the ratio of heads of livestock to acreage. The method entails certain conveniences: no moving animals from pasture to pasture; less monitoring pasture health; and less infrastructure like fences and water.
Rotational Grazing, which varies in layout and intensity, is the practice of dividing pasture into two or more cells (called “paddocks”), stocking one at a high rate, and then rotating the livestock to a new pasture once the forage in the present paddock has been grazed to an appropriate degree. The time spent in each paddock is relatively low, but due to the high stocking rates, impact on the pasture is relatively high: soil is trampled; manure is deposited; and forage is grazed rapidly.
Sometimes Rotational Grazing is practiced in such a way that it utilizes less than 7 paddocks and grazing periods of one to two weeks. This method retains some of the benefits of resting periods (as well as pasture “impact”, as we will discuss later), but the amount of time spent in the paddocks in relation to the stocking rate may be too long, resulting in overgrazing and pasture destruction. True Rotational Grazing (sometimes called “Intensive Rotational Grazing” to distinguish it) usually utilizes more than 7 (sometimes many) paddocks and grazing periods of between less than one week and half a day. If done carefully, this decreased grazing time / increased stocking rate can ensure that the pasture is not overgrazed, and increase the “impact” of livestock on the pasture.
If designing for optimal animal health, pasture integrity, and economic productivity means designing for optimal grazing pressure dynamics, then there are many situations in which Intensive Rotational Grazing is a superior alternative to Continuous Grazing. Why is this? The answer requires understanding how the naturally occurring pasture ecosystem sustains its own health and the health of grazing herbivores in a natural, unmanaged system.
The Pasture Ecosystem
Pasture ecosystems are evolved to work in coordination with intensive periods of herbivorous grazing. Animals move continuously through open pastures in large herds; they move continuously to find new forage; they herd together into large masses to reduce their individual vulnerability and protect themselves from predators. Thier density in this configuration has a high impact on the pasture. They graze preferred species down to a minimum; their hooves stamp depressions in the ground and upturn flora; and they deposit heavy amounts of urine and manure onto the ground. They must then move on to new pasture because their activity depletes the available forage.
But their intensive migratory grazing habits do more than deplete the pasture. They are also essential to maintaining it as pasture per se. Without these regular intervals of “destruction”, which create soil conditions wherein earlier pioneer-type species thrive, plant succession would take it’s course and the pasture would steadily progress into a more advanced stage of succession that favors plant species less favorable to the herbivorous diet – woodier, larger volume species with less nutritional concentration.
When animals move through the pasture and interrupt this process of succession, they perform several key functions for maintaining optimal grazing conditions on future visits. Thier grazing of preferred species prevents those species from maturing. Because grasses and forbs posses the most favorable balance between volume and nutritional density at an early stage of maturity, this ensures optimal forage nutrition for later grazing events (albeit not typically within the same herd).
Thier stamping of depressions into the ground and upturning of flora creates the ideal context for the establishment of the seedlings of preferred forage species. One example of this is the fact that some seeds will only germinate when a slight crushing pressure has been applied, for example by a hoof. Another example is that hoof depressions act as tiny microclimates, perfect for germinating seeds. They collect water; they hold the seed below the path of wind, protecting it from drying and cooling exposure; and they provide a slightly warmer ambient temperature than the soil surface.
Lastly, animals promote soil fertility by depositing urine and manure onto the pasture. These depositions comes in very high quantities, which would be detrimental if sustained for a protracted period of time, but because of the punctuated nature of the grazing events, the depositions have time to decompose before another pass is made.
This system is complex and resilient to variation. But it is adapted to work on the basic model of: short, intensive, punctuated grazing events; periodic pasture impaction; forage depletion and fertility deposition; perpetual cycles of early stage succession.
The Problem with Continuous Grazing
The practice of grazing animals continuously on a single pasture for the entire growing season subverts this naturally healthy and sustainable system at multiple levels and as a consequence creates multiple problems for itself, resulting in more inputs and less productivity. Continuous Grazing is inherently susceptible to the following shortcomings, even though variations in continuous grazing management systems exist that are more or less appropriate.
While the stamping of depressions and upturning of flora can be a rejuvenating force as a punctuated event, if done continually it will destroy pasture altogether. “Soil” is not dirt. Soil is a living organism, comprised of vast networks other other organisms living in a delicate balance. The existence of soil is premised on the continuity of conditions that preserve these life forms. If livestock are allowed to consume, trample, and foul all of the grasses growing in a pasture, rapidly or slowly, these conditions are threatened. The cycle of grasses growing, dying, decomposing, and regrowing is retarded, and too little of the right kinds of organic matter may be returned to the soil, which will further retard the growth of new grass. If soil is left exposed without a protective layer of grass, elements like wind and rain will directly impact the soil, killing delicate microorganisms which are not adapted to above ground conditions. As microorganisms die and the cycle of plant life is retarded, organic matter steadily disappeares from the soil. Gradually, the soil is reduced to dirt, the dead mineral component of soil, and the ecosystem capable of sustaining healthy forage is lost. Without plant root systems to hold the soil in place, soil structure may degrade so badly that the dirt succombs to eorison and gets carried down the watershed in rain events.
Over Grazing and Spot Grazing
Overgrazing and spot grazing both occur when livestock doesn’t have access to the right kind of forage for the right amount of time.
Overgrazing is the depletion of forage to a critically low level due to unsustainable grazing intensity. This can result from stocking rates being too high or the grazing event being too long. Because forage yields in a pasture vary throughout the growing season, grazing intensity will fluctuate (even if the intake of the animals remains the same) and will sometimes be too low, sometimes to high, making it very difficult ensure optimal grazing pressure.
Spot grazing results from the natural and desirable tendency of livestock to seek out the forage they prefer and pass over the ones they don’t, called “selective grazing”. If this process goes on continually uninterrupted, desirable species will be depleted too badly to recover and undesirable species will flourish, on account both of not being eaten and to having continually depleted competition. In this way the desirable pasture species are gradually weeded out and replaced with undesirable species. This situation can lead to overgrazing, because only the areas that retain desirable forage are grazed while the undesirable sections go ungrazed. Due to the decreasing area of the sections containing desirable forage, the stocking load becomes increasingly too great for the desirable sections and the forage is depleted.
Overgrazing contributes to erosion which can have long term negative impacts on the viability of pasture land for forage. It also directly impacts the health and productivity of livestock due to supplying inadequate nutrition.
Pasture fouling is a similar problem to overgrazing, and also occurs when stocking rates are too high or the grazing event is too long. Except, instead of depleting the pasture of foragable plant species, livestock deposit too much of the ingested forage, or urine and manure, back onto the pasture. If the pasture has no time to rest between deposition events of a certain scale, the soil conditions will become unfavorable to the growth of forage species, due for example to too much nitrogen. As the soil conditions become less favorable, the pasture’s ability to break down manure decreases, causing the conditions to become worse, and so on.
In addition to the fouling and loss of available forage, this situation can result in the spread of disease and parasites in livestock. Traditional wisdom says that about one cycle of the moon (~30 days) is the minimum time period of time to ensure that parasites deposited onto the pasture via manure are dead. This is also about how long it will take for manure to decompose to a soil-like state. Continually grazing animals in the same pastures, especially if stocking rates are too high, means continually grazing them in among 30 days worth of manure deposition, which, it seems reasonable to me, makes them roughly 30 times more vulnerable to parasites than they would be if they grazed on a fresh pasture every day.
It is important that all pastures be given some “rest” time. Ideally, animals would begin grazing a pasture when plants are 6 to 10 inches tall and removed when plants are no less than 3 inches tall. These heights are somewhat dependent on forage species. The vegetative period of growth of a species is the ideal time for grazing. Overgrazing can cause muddy conditions, erosion, killing desired pasture species and allowing for the introduction of weeds that tolerate compacted soils. On hilly land especially, rainwater runoff high in nutrients from animal feces and sediment will cause downstream pollution. Undergrazing is also undesirable as animals are likely to graze selectively, allowing less desirable plants to outcompete desired ones. Undergrazed pastures require more frequent mowing to keep undesirable plants in check, and especially to keep those plants from going to seed and spreading further. (Source)
Working With Nature: Intensive Rotational Grazing
The body of research related to better understanding how the cycles present in a naturally occurring pasture ecosystem can be appropriated for for profitable farm and ranch enterprises often goes by the names “Holistic Planned Grazing” or “Holistic Rangeland Management”. Some of the principal researchers in this field are Allan Savory, Darren Doherty, Joel Salatin.
The practice of Intensive Rotational Grazing is a variety of this holistic approach, and seeks to replicate short, intensive, punctuated grazing events; periodic pasture impaction; forage depletion and fertility deposition; and perpetual cycles of early stage succession. Through working with nature in this way, Intensive Rotational Grazing can circumvent many of the problems associated with continuous grazing.
Like in a natural pasture ecosystem, intense, periodic grazing events, while they have a strong impact on the pasture in the short term, actually help to regenerate and maintain optimal pasture health in the long term. While they animals are on the pasture briefly but densely, they graze closely, stamp depressions in the soil, upturn flora, and defecate. This works to clear the pasture of forage species before they become mature, create microclimates for new plants to germinate in, and fertilize the soil. Then, instead of continuing to impact the soil too greatly through overly prolonged stamping or grazing, they are moved on to a new paddock, allowing manure to decompose and integrate into the soil and the new plants to grow to an optimal grazing height. These new plants help hold the soil together and promote organic matter, and consequently soil integrity is maintained (which feeds back into the flourishing of pasture species).
Livestock are kept in the paddock with a high enough stocking rate that available forage is consumed with uniformity. Since more of the spectrum of more preferred to less preferred forage is consumed, a better balance of forage plant species is maintained, ensuring a healthier diet for the animals. Thus the nutritional problems of selective grazing and sport grazing are overcome.
Subdividing pasture and rotating animals encourages livestock to eat a wider variety of plants. To maximize grazing efficiency, use a very high animal density for a very short time (intensive grazing). In a large pasture animals have more grazing options and can be very choosy in plant selection. In this case, choosiness is specific to livestock species. Goats for example, will eat more woody plants than horses will. Pastures will recover while animals are moved elsewhere, and when necessary, mowing can be used to eliminate tall weeds when animals are moved out. Ideally, at least four pastures are involved in rotational grazing. Pasture recovery typically takes from 2 to 6 weeks. Rainfall, temperature, and soil fertility, as well as grazing intensity, will influence rate of pasture recovery. (Source)
Disease and parasite vulnerability is also mitigated by Intensive Rotational Grazing. 30-60 days is about how much time is required to ensure the growth of forage to optimal grazing height and about how long manure will take to decompose. It is also the amount of time it takes for parasites and their eggs to die if they are living outside of a host. By removing livestock from pastures that contain their own manure, you can ensure that livestock are consuming any of their own waste, and therefore interrupt the cycle by which parasites breed. This strategy can be bolstered by “animal succession”, a practice which conforms excellently with rotational grazing.
Animal succession is the practice of rotating different species of animal through the same pastures after one another in order to promote mutually beneficial relationships. One benefit is that the different animals have different diets and so will deplete different pasture resources. This allows the same pasture to feed multiple groups of livestock without a decrease in productivity, and can even have the regenerative effect of managing the prevalence of certain plant species. For example, ducks can be rotated through a pasture after cows to capture the energy stored in the slugs and insects, without lacking anything taken by the cows, or detracting from future grass crop for the cows. Another benefit of animal succession is the management of parasites. Most parasites that prey on one type of animal cannot prey on another, and sometimes different animals are even predators of other animals parasites. By rotating one animal through a pasture after another, you can suppress the parasite population for when the first animals make their next pass, and you can also supplement the diet of the second animals. For example, chickens can be rotated through a pasture after goats, during which time they will consume many of the parasites left behind in the goat manure , feeding themselves and reducing the parasite population for the goats on the next pass.
The rationale for this mixed-species management-intensive grazing plan is as follows: Goats and chickens are each vulnerable to different sets of parasites. Parasite load on pasture is a big problem for goats. Chickens, however, can consume goat parasites with impunity, and in fact actually benefit from being grazed on a pasture with mammal manure and the insects that feed on it, which in turn make good food for the chickens. For their part, the goats benefit from decreased parasite load and fewer flies. Both goats and chickens benefit from mixed pasture and browse although in different ways—goats prefer to eat shrub and tree leaves and bark, while chickens prefer the growing tips of new grass. But goats also eat some grass and forbs and chickens can benefit from the fruit and seeds of many shrubs. An ecosystem consisting of mixed grasses, forbs, and woody shrubs and trees tends to be more resilient and productive when impacted by animals than an ecosystem of grass alone. This is especially true when plants are selected with care for their mutually beneficial interactions. Animal grazing in such ecosystems benefits plants, animals, and soils most when animals are moved through the pasture, allowing enough time for the plants to rest and recover before they face grazing and browsing again. The benefits of management-intensive grazing are widely accepted and include more robust animal health as well as decreased carbon emissions and even carbon sequestration in soils. Factors to consider in developing a schedule for goat/chicken pasture rotation include grass and forb regrowth periods as well as fly larva and other parasite hatch cycles, factors that will vary depending on climate, plant selection, and season of the year. In some regions a combination of rotation and mixture of plant species may reduce the need for supplemental feed to virtually none. (Source)
For Intensive Rotational Grazing, grazing begins when pasture is at it’s optimum maturity (5-8 inches depending on species) and animals should be moved when pasture is grazed to its minimum height (1-3 inches depending on species). Since only one cell of the pasture is being utilized at a time, each other cell is given a rest and recovery period, allowing forage to regrow. Pasture rest time is determined by forage growth rate, which will depend on climate, season, and species. Typically, this is somewhere between 30-60 days. Pastures should be clipped if they are not utilized during this window to ensure optimal maturity when the animals arrive. Since forage growth rate depends on localized soil fertility, microclimate, and other factors, pre-planned rotational schemes should only be used in combination with careful observation to ensure that pastures are only utilized once they have been allowed to regenerate.
Pasture experts talk about the S-curve. This refers to changes in the rate of growth of the stand of plants. When plants are very small, they put on only a small amount of biomass per time. But at a certain size, the amount they add each day increases rapidly. Of course any exponential rate of growth must end, and so growth slows and halts as the plants reach maturity. S-curves are common in all biological growth systems. What good pasture management does is keep animals eating much of the pasture before it reaches the mature, no growth stage, but not so much that the pasture has a very long recovery time because the plants have been eaten back to the slow-growth portion of the S-curve. (Source)
Back to our example of the flock of 1000 sheep…If the flock needs to eat 4500 lbs of dry matter in a day we need a paddock with around twice this amount, or 9000 lbs. Since each acre has a standing dry biomass of about 3000 lbs when it’s at the top of Stage 2 in the S-curve, a 3 acre paddock will comfortably feed those 1000 sheep for a day. This is good forage management and gives us the ability to grow high quality lambs as quickly as possible during the main months of productivity, which here in Oregon is April through October. But it also does something else very important to the soil. As the plants grow, are eaten just right, and regrow again—a cycle that repeats up to seven times per year—the soil organic matter (SOM) increases. Instead of buying in tons of compost per acre, at a cost of many hundreds of dollars, our pasture produces tons of soil organic matter per acre. SOM does many things to improve soil quality, including increasing water holding capacity and improving soil tilth, and, as I’ll explain in Part 2 of this post, it stores nutrients that become available to plant roots through a transformation termed mineralization. (Source)
Paddock size should be calculated based on the relationship between animal type, stocking rate, grazing period. This chart provides estimates for paddock size based on these factors.
Paddock number should be calculated based on the relationship between animal grazing period and maximum rest time. The formula for this is: paddock number = (maximum summer rest period ÷ grazing period) + one. (Source)