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By Sandy Smart
May 2021
Rotational grazing has become an important management tool in the grazing livestock industry. This practice has been tested on tens of thousands of farms and ranches for over 50 years. The benefits it provides have been examined by range scientists, land management agencies, conservationists, and policy developers. In this article, I will provide an explanation of the efficiencies of rotational grazing from a spatial and harvest efficiency perspective. I want to be clear that the ability to increase the stocking rate is just one of many benefits of rotational grazing. The key word “ability” also implies that the grazer knows how to implement and monitor an effective rotation without causing negative consequences to animal performance and the plant community. Gaining knowledge (attending a grazing school), getting experience (trying it out slowly), and seeking assistance (NRCS, Extension, or SDGC Mentor) will help you avoid costly mistakes. Let’s look at the following to unravel the efficiencies found in rotational grazing.
Spatial Efficiency
Growing up on a small farm in Wisconsin, I did not have an appreciation for the large landscapes of the Great Plains and western U.S. One of my fondest memories from childhood was when our family would travel to California to visit my Grandma. As a youngster, seeing the change in the patchwork of crop fields expand into a vast grassland after crossing the Missouri River at Chamberlain, SD, on I-90 was a vivid memory of mine, even 50 years later. In the Great Plains, it is easy to see how the livestock stuck close to riparian areas and avoided grazing steep hillsides in pastures in excess of 1000 acres. Today, as a range scientist who has worked on grazing research for over 30 years, I have seen how reducing the pasture size increases the spatial efficiencies of grazing. Mapping the utilization of grazing livestock is the best way to observe this phenomenon. At the former SDSU Antelope research station, we had a 1,100-acre pasture that was grazed more heavily near water sources and hardly touched at all in other places. Jim Gerrish once gave a presentation at the Annual Winter Road Show where he showed this on a ranch in Montana. Through cross-fencing and water development the rancher was able to double the animal unit days on a winter range unit because they evened out the grazing distribution. I have heard this same story from many South Dakota ranchers. Unfortunately, very few scientific studies have been published to support these observations. The main reason for this lack of scientific validation is because university and USDA Agricultural Research Service research stations are generally small in size and replicating large pastures is not practical or economical. In 2013, research conducted by one of my former graduate students, using remote sensing techniques, showed how cross fencing and water development on the Buffalo Gap National Grassland near Wall, SD, improved the vegetation in riparian areas and focused the grazing in the surrounding uplands (Rangeland Ecology and Management 66:479-486). Additional articles by Dr. Richard Teague from Texas A&M University conducted at the ranch scale also support these spatial efficiencies.
Harvest Efficiency
Harvest efficiency is defined as the percentage of the annual forage produced that is consumed by the grazing livestock. Range scientists in the early years of our profession focused on setting the stocking rate such that it would achieve 50% utilization (consumption + trampling + loss by wildlife and insects + senescence) or the old adage “take half leave half.” I published a paper in 2010 that showed that the harvest efficiency from continuous season-long grazing was 25% when targeting 50% utilization (Rangeland Ecology and Management 63:397-406). This means an additional 25% was lost due to trampling, wildlife, insects, and senescence. Rotational grazing, which intensifies the stocking density, decreases the length of the grazing period, and increases the rest period in a given pasture. Research from NDSU on a long-term study comparing a 320-acre season-long continuously grazed pasture to a 320-acre rotationally grazed pasture (4-pasture twice over rotation) revealed that both pastures produced the same amount of forage, had the same amount of utilization, and had the same calf averaged daily gain, but the rotationally grazed pasture supported 44% more cow-calf pairs and hence 17.8 lbs more beef/acre than the continuously grazed pasture. When I calculated the harvest efficiency (percent consumption), the rotationally grazed pasture had a 40% harvest efficiency with only 15% loss due to trampling, wildlife, insects, and senescence. The season-long continuously grazed pasture had a 25% harvest efficiency and 29% loss through trampling, wildlife, insects, and senescence. A Nebraska extension report (EC 86-113-C) published in 1986, basically shares the same concept. Under a simple rotation, you can plan for a 30% harvest efficiency, 25% loss, and 45% left for plant vigor. Under an intensive rotation, you can plan for a 40% harvest efficiency, 20% loss, and 40% for plant vigor. Range specialists assume you can leave less residual (percent needed for plant vigor) using a rotation because you provide a longer rest period (without stress from grazing).
Planning
For grazing planning purposes, the stocking rate is calculated based on the amount of annual forage produced and multiplied by the harvest efficiency. We assume an animal unit consumes enough forage equivalent to 2.6% of their body weight to meet their daily requirements. This of course is an average. Cattle consume less when the forage is more mature and their physiological status is less demanding (non-lactating and in the first or second trimester of pregnancy) and more forage when it is vegetative and when they are lactating. Thus one animal unit month (AUM) is equivalent to 780 lbs of oven dried forage or about 912 lbs of air dried forage. For example, if a 1000 acre season-long grazed pasture produces 1800 lbs/acre (air dry) of forage we would multiple it by 25% harvest efficiency and divide by 912 lbs/AUM to get a stocking rate of 490 AUM (1800 x 0.25 x 1000 ÷ 912). If we cross fence the same pasture into four 250-acre paddocks and add a water tank in the middle, we could safely plan for a harvest efficiency of 30% giving us 592 AUM or an increase in 102 AUM. This 5% increase in harvest efficiency translates to a 20.8% increase in carrying capacity. If this same pasture was divided into eight paddocks (125 acres each) and we added a second water tank, we could safely plan for 35% harvest efficiency. This would translate to 690 AUM or a 40.8% increase in carrying capacity. These two efficiencies (spatial and harvest) are the mechanisms which allow the producer to increase the carrying capacity using rotational grazing without the adverse effects of overgrazing (high utilization) which usually accompanies season-long continuous grazing. The increase in ability to move livestock and increase carrying capacity should be used holistically to support synergies in the system.
A WORD OF CAUTION
It is easy to plan something out on paper and altogether different to execute it in real life. Rotational grazing is a science and an art. Deciding how long to graze and when to move livestock is dependent on many factors. The most practical thing you can do is attend an SD Grazing School and learn about grazing. Also, start out slow. Don’t design and invest in a cross-fencing and water development project that is too much for you to handle. Experienced grazers will say that they started out by splitting the pasture in half and trying that out. Then they would split the halves again, and again, etc. Thus, you should also not increase your stocking rate right away until you notice you have more grass available (based on the spatial and harvest efficiency principles).