Grazing & Pasture Management to Avoid Plant Toxicity

    There are a number of practices that can reduce the occurrence of plant poisonings in equines.  The most common approaches among range and livestock managers to reduce losses from plant toxicity are to: change the plant community, change the grazing animal, or change grazing management strategies. 

    Most range and pasture communities contain a few toxic plants but with appropriate grazing and pasture management, these plants are not necessarily a danger to equines.  Most horse pasture recommendations are based on other grazing livestock species and not tailored to the needs and behavior of equines.  Understanding grazing behavior of equines and knowledge of good forage management are the keys to providing healthy horse pastures and ranges with adequate forage.

• Rotation of pasture or animals

Adding a second herbivore species often improves pasture quality.  The feeding niches of grazing livestock differ, and there is a niche differentiation in foraging behavior between species.  Species of grazing herbivores exhibit differing temporal and spacial patterns of use of forage and forage areas.  Horses tend to prefer low slopes and wet depressions, while cattle will more readily graze open flats.  Therefore, changing the grazing animal may be the best approach in some situations, as in the case of infestations of tansy ragwort, which sheep can ingest without toxic effect.  In one study, quackgrass (Elymus repens) populations were abundant in areas frequented by equines, but decreased after subsequent grazing by cattle.  Constant grazing by equines may reduce species richness and diversity due to their habit of grazing slowly and moving less than other herbivores.  

 

However, for many horse owners, changing the grazing animal is not an option, adding another herbivore species is difficult, and the expense of changing the plant community may be prohibitive.  Therefore, changing grazing management is the option most suited for small horse pastures. 

• Pasture condition and health

The first factor must be the overall condition of the pasture: poor quality forage in pastures very frequently result in some type of problem in the resident livestock, whether the outcome is lowered physical condition and body weight, susceptibility to toxins in plants, or other health or behavioral issues.  Under-managed pastures result in higher densities of weeds and unpalatable plants, and don’t supply sufficient high quality forage.  The densities of buttercups and purslane (Portulaca oleracea) decreased when the stocking rates were lowered.

 

Low quality forage also significantly affects body condition, which has an impact on the toxicity of ingested plants on a grazing animal.  Lowered nutritional states of animals may result in an increase of toxic effects and decreased rates of detoxification.  Therefore toxic plants are potentially more dangerous to equines with poor body condition.  In addition, better nutritional condition can result in higher intake of toxic foods without negative repercussions.  If allowed to select a varied diet, animals have a lower chance of experiencing a toxic event, because eating a variety of plants spreads the toxins they contain over several detoxification systems, and because of interactions among allelochemicals that reduces toxicity.

 

Selective grazing is the first line of defense that an animal has against toxic plants.  While there is a natural tendency to select diets composed of several plant species, grazing preferences often depend on the nutritional state of the animal.  Normally, grazing animals avoid toxicity by limiting consumption of a toxic plant, allowing time for detoxification.  Animals also limit consumption of toxins by selecting parts of plants that contain lower concentrations of toxins. 

 

However, in depleted pastures, hungry horses may eat plants they would not if provided with ample, preferred forage.  Food intake for equines does not vary with decrease in diet quality, and when food is sparse horses widen their diets considerably.  While continued consumption of a toxic plant may result in tolerance to it, more often the affects of many toxins are cumulative, with the animal becoming progressively more poisoned, with no appearance of clinical signs for months or years.

 

If pastures contain too little forage or inferior quality forage, selectivity may result in stands with monocultures of less palatable, more toxic plants.  Grass species that tolerate frequent grazing are critical in horse pastures.  Proper stocking rates have a significant effect on the amounts and types of forages available per animal.  Species richness and diversity decreases in areas with heavy equine latrine use.

• Stocking rate 

Appropriate stocking rate is affected by factors such as soil moisture and type, climate and differences between seasons, size of horses, species of forage, and length of grazing season.  Overstocked pastures often undergo a great deal of trampling, which leads to excessive soil evaporation.  The development of grazing management strategies, particularly stocking rate, is a necessary step to ensure proper pasture usage and equine health.  

• Water and salt

Providing essential nutrients is imperative: for instance, making salt available so that animals do not develop a preference for and seek out higher sodium forage. 

 

Ensuring proper water quality and quantity is vital, since limited water availability can lead to greater toxic effects, and some toxic events may be avoided entirely with adequate water. 

• Pasture or paddock size

Boredom in animals can contribute to the likelihood that they will ingest a plant that otherwise would hold no appeal for them.  Horses left alone, particularly in smaller areas without available feed, will often nibble or chew on whatever is available to alleviate their boredom.  Therefore, it's smart practice to keep corrals and dry lots free of toxic plants.

• Weeds management

Weeds management is necessary, as overgrazing often results in higher populations of toxic invasive weeds, such as milkweed.  Plant diversity often increases following grazing management.  In one study, the number of plant species in California rangelands increased by 35% after biological control methods were applied to St. Johnswort (Hypericum perforatum).  Pastures should be examined in late summer, since frequently broadleaf weeds remain green long after other, more nutritious, forages have been depleted.  In the late summer season, overstocked pastures result in low densities of palatable grasses and higher densities of weed species. 

• In summation:

Attention must be paid to all pasture conditions: soil condition, plant communities, water, and forage availability, as well as to the physical and nutritional condition of the equines grazing it, if poisoning by toxic plants is to be avoided.

For a good checklist and set of guidelines, see the Healthy Horses, Healthy Land Self-Assessment and Guide for Horse Owners and Boarders link on the resources page.

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