Written collaboratively by Pete Bauman, Karla Hernandez and Sandy Smart.
Grassland fertilization is a topic of much interest and debate among grassland managers of all walks. From livestock managers to hay producers, ecologists and fertilizer salesmen—opinions on the value of fertilization are not in short supply. What can be hard to find in popular media are fertilization effects in relation to ecology, economics and long-term sustainability of grassland systems. The scientific literature has much to offer on this topic, and we hope to clarify some of that information here.
This article is the first in a series of six focused on helping producers and managers understand the pros and cons of grassland fertilization in relation to native and non-native grasses in planted fields, such as Conservation Reserve Program (CRP), tame or go-back pastures, grass hayfields and native grasslands/pastures. In this series, we will include information on fertilization trials and economic data relative to the Northern Plains, but first it is important to understand some of the basic terminology and information related to fertilization.
For general purposes, we are talking about commercially available fertilizers with various ratios of Nitrogen (N, often as urea and ammonia), Phosphorus (P, in the form of P2O5), and Potassium (K, in the form of K2O). The label refers to as the ratio (percentage) of N-P-K in the fertilizer and is represented in numerical form, such as 46-10-0. While there are a few more technical parts of this formula depending on the source of the components, the basic premise is that the ratio represents the percentage of the components by weight (46% N, 10% P2O5 and 0% K2O). For example, to achieve an application rate of 100 pounds of nitrogen per-acre, one would have to apply a total of 217 pounds of 46-0-0 fertilizer (i.e. 100 pounds ÷ 0.46 = 217 pounds).
Generally, a term used in regard to adding nutrients to a system or where nutrients are in rich supply. While generally assumed to be a good thing, additional nutrients are not always positive. Eutrophication can have negative impacts on terrestrial, freshwater and marine ecosystems. Think of the Gulf of Mexico and the tons of agricultural and urban fertilizers that have created the Dead Zone, an area roughly the size of Connecticut at the mouth of the Mississippi River where life is essentially absent. A more-local example would be prairie pothole lakes with poor drainage. Excessive sediment in the bottom of these lakes from erosion leads to eutrophication. Another would be manure and urine deposition in a small stock dam, which could enhance algae blooms. While not on the same scale as the Gulf, eutrophication in grasslands resulting from fertilization can have locally negative effects, both within the grassland community and through translocation of the applied fertilizer to non-target areas through runoff, erosion or atmospheric movement.
When discussing nitrogen fertilization, it is easy to become confused on the source of the nutrients. In a traditional grazing system, the livestock consume the vegetation and excrete the waste in the form of urine and manure. Nitrogen in the manure may evaporate unless recycled by insects while nitrogen in the urine soaks into the ground. Additional nitrogen is gained from the atmosphere or through nitrogen-fixing plants (legumes). While somewhat of an oversimplification, this is essentially the natural cycle of nitrogen. Therefore, artificial fertilization potentially increases overall nitrogen in the system.
Competition among plants in low-diversity systems and high-diversity systems can vary a great deal in relation to artificial nitrogen fertilization. In healthy native systems, competition for limited resources is relatively high, and thus plants fill many small niches within the environment, leading to a great deal of plant diversity across a pasture. Soil type, soil nutrients, light, water and space are all competed for; and because of these competition factors, plants that are able to compete well for the limited resources thrive. In less-complex or less-diverse systems, competition for resources is less intense due to fewer plants competing for the same resources. When a limited resource, such as nitrogen, is made increasingly available through artificial fertilization, the competitive factor is reduced, because the nutrient is now ‘easier’ to acquire, and thus plants that are well positioned to take advantage of the newly available nutrient will flourish. Often, species that flourish under nitrogen fertilization tend to be the non-native, cool-season grasses, such as smooth bromegrass and Kentucky bluegrass, as well as some of the invasive, weedy broadleaf plants.
Simply increasing productivity can be assumed as an economic benefit. However, the ratio of inputs or investments to profitability must be considered in the context of the availability of the final product.
For instance, if artificial fertilization of a hayfield or a pasture increases productivity from 1,000 pounds/acre to 2,000 pounds/acre, one might assume an economic benefit or profit.
However, if that yield increase required 100 pounds/acre of N, the manager must determine if the ½ ton yield increase was actually profitable based on the cost of the fertilizer application along with the value, volume and accessible portion of the crop produced (harvest efficiency).
Harvest efficiency is the amount of the given grass product that actually can be harvested (utilized).
In grazing systems, harvest efficiency may range from 25% to possibly as high as 50% in intensively managed systems. However, at typical 25% harvest efficiency, the animal only has access to 250 pounds of the original 1,000-pound production increase from the example above. Essentially, this necessary adjustment means that the dollars spent on nitrogen fertilization only actually results in a 25% increase in total production that counts toward true ‘profit’… down the throat of the animal.
In low-diversity grass hayfields, harvest efficiency by cutting may be as high as 80% or more, and nitrogen fertilization may make economic sense in certain cases.
Management and Vegetation Health
It is not uncommon for producers most interested in fertilization to be struggling with the perception of under-producing and weedy pastures… as compared to their neighbors.
While environmental conditions, such as location, precipitation and soils, can contribute to a sense of ‘under productivity,’ the real culprit to the lack of production is more likely historic and/or current management. Simply stated, a plant community that is overharvested, either through continuous grazing or intensive haying, simply cannot sustain a healthy root system capable of maximizing uptake of available nutrients.
In a heavily grazed native pasture system, it is unlikely that artificial fertilization would have any net benefit in this scenario, and, in fact, fertilization may be a very poor management decision, because it may simply instigate the onset of undesirable species.
In a tame pasture or grass hayfield, assuming the species composition is such where additional nitrogen would be beneficial, the manager may need to adjust harvest protocols (haying or grazing) to first allow the plant community to recover and build root health prior to investing into additional nitrogen. Nitrogen fertilization under these conditions may improve production, but only if the potential of the plant community to utilize the fertilizer is improved. In reality, nitrogen fertilization will likely only feed the undesirable species that already occur, perpetuating the existing problems.
Resources and References:
- Nutrient Network: Includes links to several articles on the topic.
- Bruckner, Monica. (2012). The Gulf of Mexico Dead Zone. Montana State University.
- Alpert, P., E. Bone, and C. Holzapfel. 2000. Invasiveness, invisibility and the role of environmental stress in the spread of non-native plants. Perspectives in plant ecology, evolution, and systematics. 3:52-66.
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- Dickson, T.L. and B.L. Foster. 2011. Fertilization decreases plant biodiversity even when light is not limiting. Ecology Letters. 14:380-388.
- Foster, B.L., and K.L. Gross. 1998. Species richness in a successional grassland: effects of nitrogen enrichment and plant litter. Ecology. 79:2593-2602.
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- Hautier, Y., P.A. Niklaus, and A. Hector. Competition for light causes plant biodiversity loss after eutrophication. 2009. Science. 324:636-638.
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- Stevens, C.J., N.B. Dise, J.O. Mountford, and D.J. Gowing. 2004. Impact of nitrogen deposition on the species richness of grasslands. Science. 303:1876-1879.