Review ArticleA Review of Equine Grazing Research Methodologies
Introduction
The American Horse Council estimates that there are over 9.2 million horses residing in the United States [1]. Forage is the backbone of the horse diet with most horses being able to receive most of their caloric requirements from forage alone [2]. Forage in the horse diet is provided in two forms, harvested forage (hay, haylage, and silage) and fresh forage (pasture). Martinson et al [3] found that 87% of horses housed in Minnesota had access to pasture during the summer, but only 24% receive their entire nutritional requirements from pasture alone. In a United Kingdom survey, 70% of responding horse owners stated that their horse had some access to pasture [4], and in New Zealand, the horse industry depends largely on pasture as the primary nutrient source for horses [5], [6].
Pastures improve horse owner profitability by providing affordable feed during the growing season [7] and also play a key role in protecting the environment. Pastures that are well managed reduce or eliminate erosion, run-off [8], [9], and weeds [10]. Almost half of horse farm operators surveyed in the Northeast United States believed that horse farms had a strong potential to impact the environment [11], [12]. Access to well managed pasture has also been linked to improved horse health, including a reduction in the incidence of colic [13], gastric ulcers [14], stereotypical behaviors [15], and osteochondritis dissecans in growing horses [16], [17].
However, horses are known to be selective grazers, especially compared with other livestock [18], [19] and when managed in continuous grazing systems. This behavior can negatively impact pasture utilization and yield. Differences in preference, defined as the behavioral response of an animal to plants when a choice is given [20], affects forage persistence because preferred species tend to be repeatedly grazed. Plant morphology, chemical composition, pubescence, and leaf coarseness are also important factors affecting preference [18], [21], [22], [23]. Forage yield and nutritive value are also important components to consider when evaluating pastures, but tend to vary among species, environmental conditions, soil type, and management practices [18], [24].
Recently, there has been a renewed interest in and increased published research on equine grazing and pasture management. In many parts of the United States, horse farms are facing regulatory issues for horses housed on pasture [11], increasing the need for research-based recommendations focused on pasture management, environmental benefits of pasture, and improved welfare of horses housed on pasture. However, differences in horse pasture-related research methodologies make it difficult to compare and apply research results across the horse industry. Therefore, the objectives of this paper are to review equine grazing research methodologies with an aim to encourage standardized pasture and grazing research procedures across the horse industry.
Section snippets
Climate and Soils Data
Forage yield and nutritive values can vary among different environments and between soil types. To help others interpret data and give additional context to results, air temperature, rainfall amount, and soil descriptions should be included when discussing equine pasture research. Most universities and research stations have weather units that collect data including daily and historic air temperature and rainfall. If research is conducted away from a university or research station site, many
Seed Bed Preparation and Grazing Management
To provide sufficient methodology for replication, seed bed preparation and grazing management should be detailed when conducting equine grazing research. This includes describing how the seed bed was prepared including herbicide application, tillage, fertility, seeding date, seeding method, seeding rates, plot or pasture size, and number of replicates. Once grazing is initiated, researchers should outline pasture management including weed control, fertility, and grazing management practices
Estimating Forage Biomass Yield
Forage biomass yield is a key characteristic to measure when conducting pasture research. Most researchers agree that yield is one of the most informative measures of pasture performance and allows livestock owners to determine stocking rates and calculate feed needs. Often yield is measured before grazing of livestock, but it can also be measured during or after grazing events. Before and after measurements allow for estimation of intake. A number of factors affect which methodology is ideal
Forage Nutritive Value
Forage nutritive value is a component of overall forage quality. Forage quality considers not only the nutritive value including crude protein and digestible energy (DE), but also factors affecting animal performance including the intake potential and antiquality factors including nitrates, alkaloids, and prussic acid that can adversely affect animal health. Forage quality and forage nutritive value are important components to consider when evaluating pastures, and are influenced by species
Plant Populations, Ground Cover, and Persistence
Plant populations, ground cover, and stand persistence are key characteristics to measure when conducting grazing research. These characteristics are commonly measured using a frequency grid, point sampling methods, visual assessments, pasture condition scoring, and the double Dominant, Abundant, Frequent, Occasional, Rare (DAFOR scale; Fig. 1). Similar to measurements of forage yield, the accuracy of techniques in predicting plant occurrence in a pasture is improved by sampling representative
Forage Maturity
The growth stage of a plant affects forage nutritive value, intake potential, and yield. Therefore, determining forage maturity at grazing or harvest is critical. There are general, and more specific, approaches for determining maturity of grasses and legumes. A general approach is to visually survey the pasture and subjectively categorize legumes as vegetative, bud, flower, first flower, seed in pods, and ripe seed [57]. For grasses, categories include vegetative, boot, anthesis, milk, and
Estimation of Forage Intake
Estimation of forage intake by grazing horses is difficult to measure. However, three primary methods have been used including difference between pre- and postgrazing herbage mass, difference in pre- and postgrazing bodyweight (BW) corrected for factors other than pasture intake, and by use of digestibility and fecal output markers.
Grazing Behavior
Grazing behavior is complex and influenced by many factors including nutritional demands, plant compounds, forage availability, social interactions, and predator risks [72]. Researchers have focused on equine grazing behavior by investigating the relationship of preference, sward height, and bite and mastication rate. However, the reasons horses display some grazing behaviors remain unclear.
Health Parameters of Grazing Horses
Grazing livestock health and pasture productivity are closely connected. When conducting grazing research, it is possible to measure several horse health parameters; however, all grazing studies should record at least horse BW and body condition score (BCS). Grazing duration, horse movement, bone development, blood metabolites, hormones, and digestive parameter are additional horse health parameters that can be measured depending on study objectives.
Evaluation of Grazing Systems
Continuous grazing is a simple method allowing animals to graze from one pasture without pasture rest and plant regrowth periods. In a continuous grazing system, horses frequently overgraze specific areas of a pasture. In a rotational grazing system, animals are moved from one pasture to the next, allowing for pasture rest and plant regrowth periods in an attempt to maximize forage production [57]. Although rotational grazing is widely used in the livestock industry to optimize pasture health
Future Technologies
Current methods to determine forage yield and nutritive value usually require destructive sampling that may not provide timely and cost-effective results that aid farmers and livestock-owners in making preharvest management decisions [114]. Recent research has investigated less labor intensive and more cost-effective approaches to assess forage yield and nutritive value in preharvest forages. Modern remote sensing technologies (i.e., measurement of canopy reflectance) have potential to
Conclusion
This review discussed common methods used in equine pasture-related research, including methods to evaluate forage, horse, and environmental parameters (Fig. 3). Movement toward standardization of research methodologies should allow for scientific comparisons and application of pasture-related research results across the horse industry.
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