Review ArticleProduction and Environmental Implications of Equine Grazing
Introduction
Horses assume a unique role in developing an ecological niche through their grazing behavior. They fertilize via urination and defecation and can modify soil structure by trampling during periods of exercise and turnout. Pasture is a good source of nutrition for the estimated 9.2 million equines in the United States, with over 90% of U.S. equine operations allowing resident horses access to pasture during the day [1], [2]. Grazing offers a myriad of benefits, however, poor manure and grazing management practices increase movement of sediment, nutrients, and pathogens into nearby surface waters causing a decrease in water quality [3]. It is important to acknowledge that these negative implications can occur when any animal is left on an area of land for too long. Horses are large bodied selective grazers; their very existence on pastures increases soil compaction, grazing intensity of forages, and manure deposition on pastures at an average rate of 18-22 kg each day. Although horse farm operators do not typically view themselves as stewards of the land, but rather as animal caretakers, almost half of horse farm operators surveyed in the northeast United States believe horse farms had a strong potential to impact the environment [4], [5]. In fact, horse farm operators in many parts of the United States are facing serious regulatory issues regarding horses housed on pasture [6]. It is therefore increasingly critical to educate horse farm operators on the best management practices (BMP) that foster management of the environment as well as health and welfare of their horses in order to bring them into compliance with regulatory agencies.
Although great advances have been made in the fields of equine nutrition, physiological, and veterinary sciences, very little scientifically based information is available regarding the environmental impacts of grazing horses on pasture. This review presents the benefits of pasture access and grazing on horse health, pasture quality and sustainability, summarizes the available information related to the environmental impact of horses grazing on pasture, and identifies knowledge gaps in the scientific literature.
Section snippets
Nutrition and Health Benefits of Pasture to the Grazing Horse
Depending on the geographical region, well-managed pastures may provide year-round ground cover and an economical feed source that can meet all of a horse's daily nutrient requirements. The digestible energy and crude protein provided by pasture can potentially meet the requirements of horses across several physiological states (e.g., maintenance, light work). Furthermore, well-managed pastures may also support mares during gestation and lactation [7]. The total acreage required per animal will
Benefits of Pasture to the Horse Farm Operator and Environment
The benefits of well-managed pasture are extensive, contributing to the well being of the horse and the land. First, a productive and properly managed pasture can meet most if not all of the horse's nutritional requirements during the grazing season, thus limiting the need to purchase additional forage or grains. Thus, supplemental hay feeding would be limited to only summer drought periods and times when access to forage is limited (winter). Furthermore, when horses spend more time on
Vegetative Damage
The act of grazing by animals, including horses, can cause irreparable damage to plant structures, and is influenced by grazing time in a restricted area, along with other factors including plant type, climate, and soil fertility. The effects of horses on pastures can be quite stunning if pastures or paddocks are overexposed to horses compared to areas where access has been limited (Fig. 1). Horses are selective grazers, consequently, they overgraze certain areas and ignore others, such as
Proportion of Time Spent Grazing While at Pasture
Horses are periodic grazers [52], and the health of their gastrointestinal tract is dependent on consumption of small, frequent meals [53]. When provided access to feed ad libitum, horses ingest feed frequently throughout the day and night. Mayes and Duncan [54] reported that equine grazing took place throughout the day, but peaks in grazing occurred soon after dawn and before dusk. Horses grazed between 63% and 75% of the daylight hours and 49% and 50% of the nighttime hours. Estimates of time
Grazing Systems
Grazing systems have been studied extensively, especially in relation to ruminant species [103]; however, few studies have been completed for comparative analysis using horses. Selection among grazing systems are often determined by the potential for pasture productivity to offset increased labor and equipment costs. Although environmental stewardship is rarely considered when designing grazing systems, such systems can greatly impact the environment. The two basic grazing systems usually
Stocking Rates
Little is known regarding proper equine stocking rates for pastures [114], [115]. Singer et al. [116] reported that appropriate stocking densities vary greatly, based on climate and region. Stocking rates are generally accepted to range from 0.4 to 0.8 ha per horse in temperate climates. In addition, forage growth rates are cyclic and have uneven growth over the course of a given grazing season, depending upon rainfall, temperatures and season. Therefore, when calculating stocking densities,
Evaluation of Horse Pastures
There are several methods to evaluate and monitor pasture conditions. The line-point transect method has been used for more than ninety years to quantify the amount and quality of plant cover at consistent and repeatable measurement points. This is accomplished by setting up specific transect lines in the desired area and choosing predetermined measurement points along those lines. At each point a vertical pin is used to identify species and measure the contact points and the parts of any
Current Management Practices
The management of grazing horses on pasture varies greatly. In the Northeast, there are a variety of management practices ranging from admittedly no management to monitoring of vegetative cover and implementation of rotational grazing [4], [5]. For example, in the North Central portion of the United States, several small acre farms are being formed from previously larger farms. Additionally, the number of small acreages in South Dakota (1-49 acres) increased 27% from 2002 to 2007 [121].
Basic
Conclusions
Research on equine pasture management and the environmental impact of grazing horses has been limited compared to other livestock species. Currently, management decisions regarding grazing systems and stocking rates are supported by data from other species with little information available for equine-specific systems. Equine-specific data are needed to develop pasture best-management practices and strategies, which can be implemented on horse farms. There is also a lack of regional research
Acknowledgments
This project was supported in part by US Department of Agriculture Multi-State project NE-1041, Environmental Impacts of Equine Operations, coordinated by Michael Westendorf, PhD. The authors thank Rachel Gilker, PhD, for consultation on soil dynamics and Cheryl Beste and Rozanne McGrath for editorial assistance.
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