Tenaya Jewell, CLS
Science Writer for Oregon Wild Horse Organization
Introduction
Various seed mixes are used on western rangelands to replant areas prone to overgrazing, wildfires, flooding and erosion. There are advantages and disadvantages to reseeding versus allowing the land to recover on its own. This article focuses on the use of alfalfa in reseeding mixes and the risks it poses to grazing animals.
Characteristics and Growing Requirements
Alfalfa is an introduced perennial herb that originated in SE Asia. Heights can reach 24 to 35 inches, with 5 to 25 erect stems per plant arising from a narrow, woody crown. In sandy soils root depth can grow 10 to 17 feet and under optimal conditions, roots can grow 23 to 30 feet (1). Varieties are available with different root system types such as: taproot, branching roots, rhizomatous, and creeping. Alfalfa is a legume; it can fix atmospheric nitrogen through a symbiotic relationship between its root nodules and Rhizobium spp. bacteria.
Alfalfa grows best in well drained soils with a pH of 6.5-7.5 that have good water holding capacity like deep, loamy soils with porous subsoils. It is intolerant of flooding, waterlogging, or poor soil drainage like in heavy clay that retains water and can lead to root rot. Alfalfa has moderate to high water requirements and needs consistent soil moisture for optimal growth; however, it will tolerate drought and is known as a good producer in dry years due to its deep root systems. The amount of precipitation alfalfa needs can vary from 12 to 30 inches, depending on soil.
Optimal growth occurs when sown on a firm seedbed. Drilling is a common, effective seeding method used in most regions. The suggested depth for planting is between 1/4 to 1/2 inch. The existence of pine and juniper litter covering buried seeds impedes seed germination (1). In ecosystems characterized by pinyon-juniper vegetation, broadcast seeding might boost germination more effectively than the drilling method. If the seeds are inoculated with Rhizobium bacteria, there's no requirement for additional nitrogen.
Benefits of Alfalfa Seeding
Most herbivores and omnivores, including all classes of livestock and big game animals consume alfalfa. Alfalfa’s early growth and ability to remain green later than grasses are valuable attributes for rehabilitation of overgrazed ranges. Several species of birds and mammals are known to use alfalfa. Birds such as sage grouse, sharptailed grouse, pheasant, California quail, gray partridge, American wigeon, mallard, and little brown crane utilize the leaves, flowers, or seeds. Alfalfa is a source of nectar and pollen for insects. Many small mammals, including jackrabbits, marmots, pocket gophers, prairie dogs, various ground squirrels, kangaroo rats, and mice graze alfalfa. Roots are consumed by pocket gophers. Alfalfa is also consumed by elk, mule deer, white-tailed deer, pronghorn, moose and bighorn sheep.
Ecological Concerns
Before introducing alfalfa, consideration of the existing range management plan is crucial. The inclusion of non-indigenous species such as alfalfa can disturb the equilibrium of native plants, potentially resulting in unforeseen ecosystem imbalance. Introduced species, when sown, can inadvertently create adverse impacts rather than positive ones, and alfalfa might not be the optimal choice. Alfalfa has been known to contribute to the suppression of native vegetation.
Most of the time, reseeding is only recommended in areas that are prone to recovery failures. Wildfire is a natural occurrence and most native plants are adapted to it. Some species will only germinate after a fire, while others benefit from the light and space that’s created in burned areas. Researchers from the US Forest Service and Northern Arizona University surveyed numerous studies conducted across the West and concluded that seeded areas were no better at preventing erosion than non-seeded areas. Even when seeded sites did produce more plant cover on the ground, it was rarely enough in the first two years to help hold soils in place. The authors wrote in their study, “this review suggests that post-fire seeding does little to protect soil in the short term, has equivocal effect on invasion of non-native species, and can have negative effects on native vegetation recovery with possible long-term ecological consequences” (2).
Alfalfa may also have a negative effect on the establishment of tree species. Alfalfa was found to contribute to the suppression of tree establishment by direct competition or by attracting rodent populations that damage tree seedlings (1).
Risks to animals
The utilization of alfalfa in rangelands comes with the potential for causing bloat in ruminant animals. Ruminants, belonging to the suborder Ruminantia within the Artiodactyla order, encompass a variety of creatures such as pronghorns, giraffes, okapis, deer, elk, chevrotains, cattle, sheep, and goats. Most ruminants possess a stomach comprising four chambers and have two-toed feet. Their upper incisors are either reduced in size or occasionally absent. Ruminants lack the ability to directly digest grasses, foliage, and other types of plant matter. Instead, their digestion takes place in a sequential manner within a multichambered stomach. In ruminants with a four-chambered stomach, the ingested plant material initially resides in the initial chamber, referred to as the rumen, where it undergoes softening. Subsequently, this material, known as cud, is regurgitated and re-chewed to further break down its cellulose content, which is inherently challenging to digest. The re-chewed cud then progresses to the other stomach chambers (the reticulum, omasum, and abomasum), where it undergoes additional digestion facilitated by essential microorganisms residing in the stomach. The digestive process of pseudoruminants is notably similar, also encompassing regurgitation and cud chewing, albeit within a stomach divided into three chambers (3). Horses do not have ruminant digestive systems, but they process some of the same substrates that ruminant animals eat. Instead of using the rumen to process cellulose, horses use their large intestines, specifically the cecum, to perform this function and thus can also be prone to bloat and/or colic.
Bloat occurs when the ruminoreticulum becomes overly distended due to the accumulation of fermentation gases. This happens as a result of an excessive buildup of gases during fermentation within the rumen. Two main types of bloat exist: frothy bloat, marked by the formation of a stable foam in the rumen, and free gas bloat, caused by either excessive gaseous compound production from fermentation or blockages preventing gas compound release. Frothy bloat is seen in cases like legume bloat. Ordinarily, these gases are expelled (belched) from the animal. If the gas is trapped in the rumen, it accumulates, causing the rumen to stretch and expand. As rumen pressure rises, it interferes with breathing, impeding the diaphragm's expansion and the creation of the negative air pressure essential for inhalation. In severe instances, this inability to inhale could lead to suffocation and, ultimately, fatality (4). This condition is most frequently observed in animals that graze on legume-rich pastures, particularly those featuring alfalfa, ladino, and red and white clovers. However, it can also arise when animals graze on young green cereal crops, rape, kale, turnips, and legume vegetable crops. Legume forages, such as clover and alfalfa, are characterized by a higher protein content and faster digestion rates. Instances of leguminous bloat are most prevalent when cattle are placed on pastures composed of rapidly growing leguminous plants in their vegetative and early bud stages (5). Bloat can be a major cause of sudden death. Grazing on bloat-prone pastures, death rates can reach 20%-30%. For animals on the range, this would be extremely problematic as medical intervention is not available. Alfalfa contains saponic glycosides, which are suspected of contributing to bloat. This risk can be reduced by planting perennial grasses to cover at least half of the site. Bloat risk is also reduced by delaying grazing until after flowering is completed (1).
Conclusion
Native plant and grass species should be prioritized and given the chance to become re- established after destruction has occurred. Introduced species like alfalfa create disturbances and can cause shifts in plant and animal populations. While alfalfa can provide forage for a number of species it also comes with health risks to grazing animals and changes the landscape in unforeseen ways.
References:
1. Sullivan, J. 1992. Medicago sativa. In: Fire Effects Information System, [online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: https: www.fs.usda.gov/database/feis/plants/forb/medsat/all.html
2. Peppin, D. Fule, P., Beyers, J., Sieg, C., Hunter, M. (2011). Does seeding after severe forest fires in western USA mitigate impacts on soils and plant communities? CEE Review, 08-023 (SR60). Collaboration for Environmental Evidence: www.environmentalevidence.org/SR60.html
3. Britannica, The Editors of Encyclopedia. "ruminant". Encyclopedia Britannica, 5 Jul. 2023: https://www.britannica.com/animal/ruminant
4. Lehmkuhler, J. Burris, R. Arnold, M. Smith, R. Lacefield, G. (2011). Managing Legume-Induced Bloat in Cattle. Cooperative Extension Service University of Kentucky College of Agriculture (ID-186).
5. Constable, P. 2022. Bloat in Ruminants. Merck Manual.
https://www.merckveterinarymanual.com/digestive-system/diseases-of-the-ruminant-forestomach/bloat-in-ruminants