Grassland Fertilization | Part 4: Native Pasture (Case Study 3)

By Pete Bauman

In the third article of the Grassland Fertilization Series we presented two case studies on fertilization of native grasslands from McPherson County and a cooperative project from Deuel, Grant, and Clark Counties. In this article we highlight a third native pasture case Study from Hamlin County.

Case Study 3, Thomas and Poinsett Area, Hamlin County:

2005-2006 Northeast SD Pasture Fertilization Trials (Smart, Smith, Lentz, and Woods. 2005-2006. Unpublished data.)

Range Science faculty at SDSU in cooperation with the Natural Resources Conservation Service and private landowners initiated this study in Hamlin County, SD in 2005 to investigate the effects of N application on pasture sites. At site 1 near Thomas, SD the pasture had never been tilled, however the portion of the pasture included in the study had shifted from a native grass-dominated community to one where introduced smooth bromegrass and Kentucky bluegrass were the predominant species. The site was managed with rotational grazing, in good to excellent condition, and had an average of six inches of residue left from the previous grazing season managed with rotational grazing. Site 2, located in the Lake Poinsett area was also native rangeland. Although this site had never been farmed, it was dominated by introduced Kentucky bluegrass and smooth bromegrass and native green needlegrass. Site 2 was managed with continuous grazing, in poor to fair condition, and had only two inches of residue left from the previous year.

Investigators applied four levels of N fertilization: control (0 N) and treatments of 45, 90, and 135 lbs./acre N. Three clipping treatments were applied to simulate various grazing scenarios. Forage production was estimated using a 0.96 ft2 circular frame with clipped samples weighed to the nearest gram, then dried in a forced air oven to determine dry matter yield. Preliminary soil N tests were conducted with a 24 inch nitrate N soil test conducted at the SDSU soil lab. Investigators were able to anticipate an increase in forage production at both sites based on initial soil tests.

Forage production improved at both sites, with increases at site 1 higher than those at site 2. Investigators speculated this was likely due to the differences in species composition and past management history. Smooth bromegrass dominated site 1 which had been grazed moderately over the previous several years. Smooth bromegrass, an introduced cool-season species, has been shown to respond favorably to N. At site 2 the dominant species was Kentucky bluegrass with lesser amounts of smooth bromegrass and green needlegrass. Site 2 had been grazed heavily over the previous several years. Therefore the potential of site 2 to respond to increasing levels of fertilizer was likely limited to the presence of less productive species with lower vigor.

Pre-treatment soil N and P levels were low according to SDSU Extension recommendations. Nitrogen levels were 8 lbs./acre at site 1 and 13 lbs./acre at site 2. Soil P levels were 3 ppm at both sites. The soil samples at the end of the growing season revealed that the N applied was used by the growing forage or lost to the system. Post-treatment soil N ranged from 9 to 10 lbs./acre at site 1 and from 6 to 7 lbs./acre at site 2. Post-treatment soil P ranged from 5 to 11 ppm and appeared to have carried over since the baseline level was 3 ppm at site 1. Soil P was unchanged at site 2.

Therefore, N use efficiency as calculated by lbs. of forage dry matter divided by lbs. of N applied was greater for site 1. Nitrogen use efficiency was greatest (52 lbs./acre of forage per 1 lbs. of N applied) at the 90-9-9 level, and lowest (29 lbs./acre of forage per 1 lbs. of N applied) at site 1 at the 135-14-14 level. At site 2, N use efficiency was greatest (20 lbs./acre of forage per 1 lbs. of N applied) at the 135-14-14 level, and lowest (7 lbs./acre of forage per 1 lbs. of N applied) at the 45-5-5 level. Inefficiency in N use at site 2 was likely related to the inherently lower producing species. If grasses are over utilized, their root system may be smaller and less able to take up nutrients. This poses a possible problem with N being lost through volatilization or leaching through the soil profile below the rooting zone.

This study illustrates that site potential is very important in considering management decisions such as fertilizing or grazing strategies. The lack of response to heavy simulated grazing management at site 2 suggests that species such as Kentucky bluegrass can handle frequent, heavy defoliation. Implementing a rotational grazing strategy at site 2 will not necessarily improve forage production because Kentucky bluegrass may not have the yield potential (photo 1). However, a higher producing site like site 1 which is dominated by smooth bromegrass has a greater potential to increase forage production when fertilizer is added and a rotational grazing strategy is used.

Photo 1: An example of a native pasture community that has suffered under poor management and is now dominated by exotic Kentucky bluegrass. In this case the plant community is simplified and there is very little root capacity to take advantage of artificial N. -

The main purpose of this study was to see if fertilizing pastures paid. The spring 2005 quote from the Hayti Farmers COOP, for 46-0-0 fertilizer was $325 a ton with an application fee of $4/acre. If 45 lbs./acre of N were applied the cost would be $16/acre, for 90 lbs./acre of N cost is $32/acre, and for 135 lbs./acre of N the cost would be $47/acre. At today’s prices, 46-0-0 fertilizer costs $455/ton ($0.49/lbs.) and would have costs $22 for 45lbs./acre, $44 for 90 lbs./acre, and $66 for 135 lbs./acre of N for x, y, and z, respectively.

At site 1 baseline yield with no fertilizer ranged from 2,000 to nearly 4,000 lbs./acre depending on the grazing strategy. At 90 lbs. N/acre of , yields ranged from about 2,000 to 2,500 lbs./acre. At the extreme, yield increased by approximately 3,500 lbs./acre under the two pasture, 3 wk. rotation system. At today’s rates of $44/acre for 90 lbs./acre N, that maximum additional forage was produced at a cost of approximately $25/ton.

At site 2 baseline yield with no fertilizer ranged from about 500 to 1,000 lbs./acre depending on grazing strategy. At 90 lbs. N/acre, yield ranged from 4,000 to nearly 7,000 lbs./acre. At the extreme, yield increased by approximately 2,000 lbs./acre under the two pasture, 3 wk. rotation system. At today’s rates of $44/acre for 90 lbs./acre N, that maximum additional forage yield was produced at a cost of approximately $44/ton.