Part 5 of the Grassland Fertilization Series discussed the effects of fertilization on native grassland plantings. This last installment addresses the options for fertilization on low diversity exotic grassland plantings.
Low diversity non-native hayfields and pastures
There exist a fair number of fields across South Dakota that were intentionally planted to low diversity non-native grasses and forbs. A common example of such fields are historic alfalfa or alfalfa/grass mix hayfields that have reverted to a grassy state. These areas are now more or less dominated by species such as smooth brome, intermediate wheatgrass, or crested wheatgrass with remnant populations of alfalfas, clovers, or other legumes. A second fairly common example of low-diversity non-native fields are expired CRP fields. These were originally established with some combination of smooth brome, intermediate wheatgrass, alfalfa, sweet clover, and possibly include some native warm season grass species.
Nitrogen applications to low-diversity non-native hayfields and pastures
Case Study 5, Leola Area, McPherson CountyIn all scenarios discussed in this series on nitrogen, the most practical grassland community in which to consider N application would likely be low-diversity non-native grasslands managed as hayfields. These are comprised of species able to utilize N and where nutrients are removed from the system when hay is harvested (photo 1).
2004 Fertilizer N and P Influence on pasture yield in McPherson County, SD (Gerwing et al. 2004).
Study 2 was conducted by SDSU researchers in 2004 on two pasture sites near Leola, McPherson County. While this study included two sites, in neither case did the study indicate whether the pastures were truly native sod. Site 1 was dominated by Kentucky bluegrass with some native and introduced broadleaf plants (please refer to Case Study 1 in this series). Site 2 was dominated by introduced species including intermediate wheatgrass, smooth bromegrass, and alfalfa, making its classification as native pasture highly suspect. Therefore, site 2 is included in this section of planted non-native pastures.
At site 2, researchers applied urea (46-0-0) and triple super phosphate (0-46-0) to the pasture. Phosphorus rates were 0 and 40 lbs./acre and at this application rate dry matter production (5,887 lbs./acre) did not increase over the control which yielded 5,609 lbs./acre with no added P. The actual increase only being 278 lbs./acre.
Application rates for N at site 2 were 0, 30, 60, 90, and 120 lbs./acre. Production was 5,167 lbs./acre without N application. Yield rose consistently with N application to a maximum of 6,994 lbs./acre produced at 120 lbs./acre of N; this was a total increase of 1,827 lbs./acre over the non-treatment base yield of 5,167 lbs./acre. Yield however, only increased significantly from 0 to 30 lbs. N/acre to 6,007 lbs./acre (840 lbs./acre over the base yield of 5,167 lbs./acre).
Interpretation: As described in Case Study 2 of this series, it is important to understand that while overall production may increase, not all of that production results in actual forage consumed by the animal. Here in Case Study 5, even at the highest N application rate (120 lbs./acre) only an additional 1,827 lbs./acre of forage was produced. At a 30% grazing harvest efficiency, livestock would consume only an additional 548 lbs./acre of forage. At today’s market prices the investment of $0.49/lbs. of N would cost approximately $59/acre in fertilizer expenses, only to achieve an additional 548 lbs./acre of forage, with an approximate cost of $215/ton.
In a hay-making situation at 80% harvest efficiency however, one might be able to bale up to 1,461 lbs./acre of the additional 1,827 lbs./acre produced at the maximum 120 lbs. N/acre rate. This would reduce investment in fertilizer for the extra forage to approximately $80/ton, which may still pose a significant financial risk in the hay market. In this case, where high N-affinity non-native species are the target, boosting production through artificial N application poses little ecological risk. One exception is the inherent risk that N fertilization may not be successful under adverse environmental conditions such as drought.
2013 Nitrogen effects on production of CRP fields planted to non-native intermediate wheatgrass, smooth brome, and alfalfa (Bauman and Hernandez).
In this study the landowner had historic CRP fields that were planted primarily to non-native intermediate wheatgrass with smooth bromegrass and alfalfa common throughout the fields. The landowner managed the grass fields nearly exclusively for hay production and wildlife habitat and was interested in the effects of N fertilization on production.
On May 13, 2013 N (46-0-0) fertilizer treatments were applied to several portions of planted hayfields at a rate of 100 lbs./acre of bulk N product, or 46 lbs. N/acre at a cost of $34.47/acre. In addition, paired portions of the fields were left unfertilized, as were all portions of the native pastures at the site.
SDSU Extension field staff was invited to conduct yield and soil assessments at the site. Samples were collected on August 5 and 8, 2013 at near peak production of the native grasses. Clippings were collected utilizing standardized methods with a 0.96 ft2 clipping ring. Samples were dried in a drying oven at SDSU and weighed on a digital gram scale. Average production of the N-fertilized plots (n=3) was 7,950 lbs./acre which did not differ from the unfertilized plots (n=8; 6,263 lbs./acre; p=0.28). The average numerical yield difference between fertilized and unfertilized was 1,687 lbs./acre. Fertilized and unfertilized plots were in all cases dominated by intermediate wheatgrass and smooth brome as would be expected in this low-diversity planted system.
Three soil core sub-samples were collected utilizing a 12” soil probe along with the vegetation samples from each of the sample areas listed above. Soil samples were analyzed for N and carbon content at the SDSU soil lab. Soil carbon averaged 42.1% in fertilized plots and was not different than the 42.2% soil carbon recorded in unfertilized plots (p=0.83). However, soil N was 0.64% in fertilized plots which differed from the 0.84% soil N recorded in unfertilized plots (p=0.02).
In addition to fertilized and unfertilized hayfield samples, SDSU Extension staff also analyzed the base production in five areas of the unfertilized native pastures. Cattle were excluded from the sample sites with grazing exclosures. The native pastures on this farm were heavily infested with non-native cool season grass species and the plant community was dominated by smooth brome and Kentucky bluegrass with intermediate wheatgrass and crested wheatgrass also common. Native cool or warm season grasses were largely non-existent except for a limited occurrence of buffalo grass and sedges in some locales. Unfertilized production from these pastures averaged 3,580 lbs./acre (n=5) and was less than unfertilized portions of the non-native hayfields, which averaged 6,263 lbs./acre (n=8) (p=0.04). This is largely due to the heavy dominance of intermediate wheatgrass in unfertilized hayfield samples which was extremely tall and robust when compared to the smooth brome/Kentucky bluegrass mix prevalent in grazed pastures.
Soil carbon and soil N were also compared between unfertilized pasture and unfertilized planted hayfields. Soil carbon averaged 42.2% in unfertilized hayfield plots and was not different from the 41.8% soil carbon recorded in unfertilized pasture plots (p=0.37). However, soil N content was 1.22% in unfertilized pasture plots which was higher than the 0.84% soil N recorded in the unfertilized hayfield plots (p=0.003).
Interpretation: While not statistically different, N-fertilized plots outperformed unfertilized plots on average by about 1,687 lbs./acre at a cost of about $34.47/acre or about $40.87/ton of additional biomass produced. At an average of roughly $100 - $120/ton value for CRP grass (August 2013), the profit margin on fertilization of this grass hay may have been approximately $60 - $80/ton or $84 - $101/acre (source: Dakota Hay Auction website. dakotahayauction.com). Hay harvest costs need also be factored into the profitability equation for a true estimation of profit potential.
These fields were managed exclusively for hay production and habitat, with hay harvest rotated through the fields on a biennial basis. It is unlikely then that litter accumulation due to higher biomass production as a result of N fertilization would constitute a major concern for this simple plant community. In this case, where non-native species with high N-affinity are the target, boosting production through artificial N application poses little ecological risk. An exception would be the inherent risk that the production benefits of N fertilization might not be realized under adverse environmental conditions such as drought. The 2013 growing season was very favorable for intermediate wheatgrass and smooth brome growth and likely both species maximized the availability of additional N.