Photo 1: An example of a healthy native plant community where artificial N would be harmful.
Photo 1: An example of a healthy native plant community where artificial N would be harmful.

In the first two articles of the Grassland Fertilization Series, we introduced the background information on the overall terms & economics, and ecological effects of nitrogen in plant communities. We encourage you to review those articles.

While nitrogen fertilization of native grasslands can potentially increase production in the short term it is generally assumed to be detrimental to the overall long-term health of the plant community. It is rare to find any grassland in South Dakota, native or otherwise, that doesn’t have some level of infestation of non-native invasive grasses such as smooth bromegrass, Kentucky bluegrass, and cheatgrass. Even our best native pastures, rangelands, and prairies suffer from at least some level of invasion. Within this reality lies a wide gradient of quality of the native grasslands that is largely influenced by past and present management.

In healthy native grasslands with only limited invasion of non-native grasses (photo 1), nitrogen fertilization will likely bolster the competitive advantage of the undesirable invasive grasses. This in turn will negatively impact the plant community likely leading to a reduction and possible elimination of certain native plants from the system over time. In addition, once the invasive species gain a competitive advantage, they will likely continue to self-propagate and gain dominance in the community, leading to additional decreases in native plant diversity and richness.

In native pastures already dominated by invasive non-native grasses such as smooth brome, nitrogen fertilization will likely increase production but may have lasting negative effects on the plant community….driving an already unbalanced system further toward single-species dominance. In turn, this simplification of the plant community can severely limit the pasture’s resiliency to changing environmental conditions such as drought, thus restricting the pasture’s use to certain seasons (photo 2).

Photo 2: An example of a native pasture plant community with a mix of native and exotic grasses where artificial N may allow the exotic grasses to outcompete the native species.

Case Studies

While there is a great deal of anecdotal information related to fertilization of grasslands in South Dakota, there is limited information from studies designed to look specifically at the effects of fertilizing native pastures. Those studies that have taken place have been relatively small, isolated, and while occasionally incorporating native pastures, the quality of those pastures has been generally degraded and/or highly invaded with cool season exotic species such as Kentucky bluegrass and smooth bromegrass. We found no studies here nitrogen fertilizer was applied to pristine native grasslands. These studies are likely lacking because those types of grasslands are often managed by individuals already aware of the harmful effects artificial nitrogen applications can have on a native plant community and thus do not/will not conduct such experiments. Part 4 of this series will highlight an additional native pasture case study that took place in Hamlin County.

Case Study 1: Leola Area, McPherson County:

Fertilizer N and P Influence on pasture yield in McPherson County, SD in 2004 (Gerwing et al. 2004).

Study 1 was conducted in 2004 by SDSU researchers on two pasture sites near Leola in McPherson County. The study did not indicate whether the pastures were truly native sod. Pasture site 1 was dominated by Kentucky bluegrass with native and introduced broadleaf plants. Pasture site 2 was dominated by all introduced species including intermediate wheatgrass, smooth bromegrass, and alfalfa making its classification as native pasture highly suspect. Therefore, site 1 is included in this section on native grasslands. (Results from site 2 are included in case study 5 in this series associated with non-native planted grasslands).

At site 1 where Kentucky bluegrass dominated the pasture, researchers fertilized with urea (46-0-0) and triple super phosphate (0-46-0). Phosphorus was applied at 40 lbs./acre and increased dry matter production from 2,580 to 3,121 lbs./acre (+540 lbs./acre). Application rates for N at site 1 were 0, 30, 60, 90, and 120 lbs./acre. Production was 2,115 lbs./acre without N application. Yield rose consistently up to a maximum of 4,706 lbs./acre where N application was 120 lbs./acre. Yield only increased between no fertilization and 30 lbs. N to 2,280 lbs./acre and then again to 4,127 lbs./acre when 90 lbs. N were applied. See the interpretation portion of Study 2 below for a discussion on valuing total forage produced against the amount of forage actually available to livestock.

Interpretation: No information was provided regarding the composition of the yield data in relation to plant species. It is assumed that with site 1 being dominated by Kentucky bluegrass, this is likely the dominant species in the yield trial. No information was provided on invasive species or post-trial plant community characteristics. It is assumed that in degraded pasture systems such as this, N application will only perpetuate the dominance of Kentucky bluegrass.

Case Study 2, Deuel, Grant, and Clark Counties:

2004-2009 Northeast SD Pasture Fertilization Trials (data derived from Maaland and Kruse 2004/2005 and Langner, McGraw, Schafer, Marxen, and Guthmiller 2009).

Study 2 was conducted by SDSU Extension in Deuel, Grant and Clark counties in 2004, 2005, and 2009 and was focused on measuring yield increases and economic return from application of N and P at various rates. The authors indicated that while these trials were conducted on what was determined to be native pastures/hay land, all sites were fairly heavily infested with non-native invasive grasses such as smooth bromegrass and Kentucky bluegrass with limited native grass component (Langner. Personal communication.). Further, these sites were selected for the study specifically because they had a heavy influence of introduced grass species that were known to respond well to N applications.

The authors indicated the following market prices at the time the study data was compiled (2009). Grass hay $75/ton, 46-0-0 N $475/ton ($0.52/lbs.) and 18-46-0 P $510/ton ($0.55/lbs.). For comparison, today’s prices are approximately $100-$120/ton for fair to good grass hay, 46-0-0 N at $455/ton ($0.49/lbs.) and 18-46-0 P is $550/ton ($0.60/lbs.). No study methodology was provided, but the investigators indicated that five trial plots were non-randomly located within each pasture (Langner. Personal communication). Plots were in close proximity to one another, and livestock was fenced out of the trial plots during the growing season.

Interpretation: Generally as fertilizer application rates increased so did overall vegetative production. Researchers indicated that while productivity increased with fertilizer applications, weed infestations also increased across all sites (Langner. Personal communication). This increase in invasive species required additional inputs associated with weed control, thus impacting the actual profitability that might have been realized at various application rates. However, weed control costs were not estimated.

Further, and perhaps more important is the need to understand that not all increased forage production results in actual forage consumed by the animal. The results of this fertilization trial are reported in the table below. The 135-60 application yielded the greatest average total lbs./acre (7,333 lbs.) and the greatest yield over the baseline control plot (5,255 lbs.). However, in a grazing situation with roughly 30% harvest efficiency, only 1,576 lbs. of the 5,225 lbs. increased production is actually consumed by the animal. In reality, the additional 1,576 lbs./acre of forage cost, at today’s fertilizer prices, $102.64/acre to produce, resulting in a final cost of feeding equivalent to roughly $130/ton. At that rate of investment, one would be better off purchasing hay than to rely on fertilizer to produce additional forage for grazing.

However, in a hay harvest situation at 80% harvest efficiency, one might be able to harvest up to 4,204 lbs./acre over the baseline at a cost of $102.64/acre. This is equivalent to about $49/ton fertilizer investment into hay. This may be financially profitable in some years, assuming the risks of weedy infestations and potential degradation of the native plant community are taken into account. Finally, it is unlikely that 80% hay harvest efficiency can be achievable on most truly native pastures due to topography, access, rocks, and other hindrances. In addition hay harvest expenses must be considered as well when determining end profit.

A total average of 3,786 lbs./acre was produced in Study 2 compared to the 4,127 lbs./acre recorded in Study 1 with the 90 lbs. of N application rate. However, in reality only an additional average of 512 lbs./acre was actually produced as available forage at a cost of $44.51/acre (today’s fertilizer cost equivalent). The final cost of the forage produced is $173/ton, significantly higher than even premium grass hay at today’s market prices.