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Choosing a grazing system: Mob vs. rotational

Eric Billman, Elle Andreen, Jessica Williamson and Kathy Soder for Progressive Forage Published on 30 August 2019

In the last several decades, two grazing management strategies have become the go-to for most pasture-based livestock operations: rotational and mob grazing.

In rotational grazing, cattle are allowed to graze a paddock for several days before moving to a new area, with a resting period of around 30 days, or until forage has regrown to an ideal height, before the next grazing rotation. In mob grazing, intense high-stocking-density grazing events allow for substantial forage accumulation of forages in later stages of maturity due to a much longer resting period but cause extensive defoliation and trampling. Neither strategy is universally superior; each offers benefits and risks depending on your operation and management.

Rotational grazing allows for frequent grazing events and high-quality forage but can put significant strain on pasture longevity if grazed too frequently. On the other hand, mob grazing provides abundant forage capable of supporting high stocking rates, but potentially results in lower forage quality due to stagnated plant growth along with lower use efficiency by grazing animals due to greater trampling of excess growth.

Identifying the right strategy for your operation can be difficult, as the success of each strategy depends on the predominant species in your pastures. Most pastures are a mixture of grasses, legumes, and forbs, and depending on how this species composition changes over time, forage availability and quality can vary substantially across the production season. Grazing is a major factor in how pasture composition changes, with some species responding better to grazing than others.

This means that how your grazing management style, rotational versus mob, affects forage yield and quality could largely be based on how your particular pasture species respond to more or less frequent grazing. While some preliminary research has been done looking at the efficacy of mob grazing, our knowledge of how perennial-based pastures typical of the eastern U.S. respond to rotational versus mob grazing is limited.

A four-year grazing study was conducted at the Hawbecker Research Farm in Bellefonte, Pennsylvania, in collaboration with Pennsylvania State University and the USDA-ARS Pasture Systems and Watershed Management Research Unit (University Park, Pennsylvania) to investigate how rotational versus mob grazing influenced pasture composition and forage yield and quality. The trial was initiated in 2014 and compared rotational grazing, typically five to six grazing episodes per year, with twice-per-year mob grazing (about 250,000 to 300,000 pounds per acre based on a rough estimate of 1,000 pounds per cow and 30-40 cows per 1/8-acre paddock). Plots were established in June 2014 with a mix of alfalfa, white clover, orchardgrass, tall fescue and buckhorn plantain. Grazing began spring of 2015 and plots were grazed during the growing season for four years: 2015, 2016, 2017 and 2018.

One of the primary questions was how total yield from a few large harvests in mob grazing would compare to small but more frequent harvests in rotational. For cumulative, whole-season dry matter (DM) yield (Figure 1), rotational grazing had slightly higher cumulative yield in the first year, and the two strategies were relatively similar in years two and three.

cumulative forage availabilty over a four-year period

By the final year, the rotational plots produced 1.3 tons per acre more than the mob-grazed plots. This suggests repeatedly subjecting pastures to long periods of growth with no grazing events, followed by intense grazing events, could negatively impact productivity in the long term. This may have been because some of the physically larger species in mob-grazed paddocks, such as alfalfa, outcompeted several other species when allowed to grow for a longer period. Findings also suggest most of the species present in this study can tolerate more frequent grazing events, and the extra forage accumulated at each harvest in mob grazing doesn’t necessarily compensate for fewer total harvests.

Another interesting finding was the variation in forage availability throughout the season in mob grazing, but not rotational grazing (Figure 2).

Per-harvest dry matter yield compared between mob and rotational grazing

Forage yield of rotational plots varied by only about 1 ton per acre between grazing events, while mob grazing plots varied by up to 2.3 tons per acre. The mob-grazed plots also provided most of their forage mass at the first grazing event, with the subsequent harvest yielding only about half the original quantity. This is an important factor to consider in implementing mob grazing as producers need to account for much lower pasture productivity later in the season. This may mean shortening time in each paddock, providing supplemental feed or only mob-grazing early in the year.

In addition, species composition differences between management strategies were observed. The rotationally-grazed plots typically had greater proportions of orchardgrass and tall fescue compared to mob-grazed, likely because these grasses respond well to repeated grazing events. For alfalfa, however, after the first year the proportion in the rotational plots began to steadily decline, reaching almost zero by year four.

Alfalfa and other large legumes were much more persistent in mob-grazed paddocks, where they did not experience such intense grazing pressure. Species more predisposed to shading, such as clover and plantain, declined to nearly zero in both systems, but did so faster under mob grazing where the canopy was not frequently opened during grazing events. Neither management strategy was consistently superior in terms of weeds or proportion of dead material.

Lastly, considerable impacts on forage quality between mob and rotationally-grazed paddocks were found. Forage acid detergent fiber (ADF) and lignin content were consistently higher under mob grazing, while total digestible nutrients (TDN) was higher in rotational. Crude protein was greater in the mob-grazed paddocks until the final year, probably due to greater persistence of alfalfa and other legumes. These differences in forage quality were expected since forages in rotational paddocks were harvested earlier in maturity, resulting in higher nutritive quality. This is another important consideration for mob grazing; while yields at each harvest may be greater, forage quality is expected to be reduced.

This study provided some insight into important questions producers face when selecting a grazing management system. First and foremost, be aware that grazing management can affect nearly everything about your pastures: forage yield, species composition, grazing interval and forage quality. Our findings suggest that rotational grazing is likely better suited for pastures based on introduced species common in the eastern U.S., as those species can tolerate frequent grazing events better than those typical of the western U.S.

While yields at each harvest were consistently higher for mob-grazed paddocks, they did not necessarily provide more total forage over the entire grazing season and may decline in productivity over time. Mob grazing also provided a less consistent and lower-quality forage supply compared to rotational. However, mob grazing could still provide a viable alternative for producers in the eastern U.S. in need of a large source of available forage. Those with substantial acreage and large herds capable of adequately using the larger yields would benefit most, provided that long-term productivity of the pasture, species diversity and declining forage quality are not major concerns.  end mark

Elle Andreen is a USDA-ARS support scientist at State College, Pennsylvania.

Jessica Williamson, Ph.D., is with Penn State Extension at Pennsylvania State University.

Kathy Soder is a USDA-ARS animal scientist at State College, Pennsylvania.

Eric Billman is a USDA-ARS Post-Doctoral Research Scholar State College, Pennsylvania. Email Eric Billman.

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