Plant and Soil Responses to Source, Rate, and Timing of Applied N for Plains Bluestem Production

Abstract

In western Oklahoma, one of the most popular grass species being used for pastureland is Plains bluestem (Bothriochloa ischaemum L.). Despite the current importance of this species, N fertilizer sources, rates, and timing of application have not been thoroughly evaluated for this species. The objective of this 3-yr study was to evaluate the effect of source, rate, and timing of applied N on Plains bluestem forage production. Two established stands of Plains bluestem in Stillwater and Bessie, OK, were selected as experimental sites. A randomized complete block design with three replications was employed at each location. Urea and ammonium nitrate were evaluated at rates of 50, 100, 200, and 400 lb N/acre. Time of fertilizer application was evaluated using late April and early May applications. Dry matter yield, crude protein (CP), and N uptake increased with applied N up to 200 lb N/acre for both sources. The maximum average (3-yr) yields were 4112 and 3370 lb dry matter/acre at Stillwater and Bessie, respectively. No effect on production was found to exist due to N source. Applying 50 and 100 lb N/acre in early May increased yields at Stillwater 25 and 14%, respectively, compared with the same rates applied in late April. This study indicated that Plains bluestem forage yield, N uptake, and CP content increased significantly when up to 200 lb N/acre was applied in the spring. Dry matter production was further increased when N was applied in early May compared with late April. Research Question What are the effects of N fertilizer source, rate, and time of application on Plains bluestem forage yield and crude protein content? Literature Summary Plains bluestem is a perennial, warm-season grass that is popular in western Oklahoma due to its aggressiveness, persistence, and drought tolerance. The species has a high livestock and forage production potential. In most instances, farmland that has been seeded to grass is N deficient due to several years of cultivation and erosion. Researchers have found N fertilization to substantially increase forage production resulting in an average of 30 lb of forage being produced per pound of N applied. Others have reported that applications of 30 lb N/acre resulted in steer gains ranging from 5.0 to 9.5 lb/yr per lb N applied. Steer gain response to N fertilization may result not only from higher forage production, but also from forage of higher crude protein content. Various researchers have found crude protein levels in bluestem hay to increase with increasing N fertilization. Nitrogen use efficiency in Plains bluestem has been reported to range between 24 and 40%. Study Description Two established stands of Plains bluestem in Stillwater and Bessie, OK, were selected as experimental sites in the spring of 1994. A randomized complete block design with three replications was employed at both sites. Plot size was 10 by 25 ft. Standing dead herbage was mowed and removed from the experimental area each year prior to fertilizer application. Nitrogen applications were made in either late April or early May using a conventional fertilizer spreader (Table 1). A 78.5 sq ft area was harvested at a height of 3.0 in. from each plot and subsampled for moisture and percentage N analysis. Percentage N was used to calculate crude protein content and forage N uptake, which was subsequently used to estimate fertilizer N recovery and N use efficiency for each treatment. Following the final harvest of 1996, duplicate soil cores were taken to a depth of 6.0 ft from each plot using a hydraulic soil probe. Soil samples were subsequently divided into increments of 0 to 6, 6 to 12, 12 to 18, 18 to 24, 24 to 36, 36 to 48, 48 to 60, and 60 to 72 in. Soil samples were air dried at ambient temperature, ground to pass a 100-mesh sieve, and analyzed, by depth, for ammonium N and nitrate N to estimate soil inorganic N accumulation. Applied Questions Does N fertilizer increase the productivity of Plains bluestem? Yes. Plains bluestem forage yield increased up to 200 lb applied N/acre. This rate produced average yields of 4112 and 3370 lb forage/acre at Stillwater and Bessie, respectively, which resulted in increases above the check (0 N applied) at each location of 116 and 207%. Which N source performs better when used on Plains bluestem? Under certain conditions, surface applied urea can hydrolyze and subsequently be lost as a gas, resulting in decreased yield. However, in this study there was no difference between ammonium nitrate and urea. Weather, soil conditions, and fertilizer cost and availability will determine which N source should be used. When is the best time to apply fertilizer to Plains bluestem? This study indicated that yields were significantly greater when N was applied in early May than when the same rate was applied in late April. These results agreed with previous work that recommended N be applied 3 to 4 wk following spring green-up. Are there any benefits of N fertilization other than increased yield? Yes, crude protein content also increased with increasing N rate. Crude protein contents, averaged over cuttings, increased from 6.5 to 9.7 %. Table 1. Treatment structure employed in established Plains bluestem pastures at Stillwater and Bessie, OK. N rate, lb/acre N source† Time of application 0 -- -- 50 AN Late April 100 AN Late April 200 AN Late April 400 AN Late April 50 UR Late April 100 UR Late April 200 UR Late April 400 UR Late April 50 UR Early Ma 100 UR Early May † AN-ammonium nitrate (34-0-0), UR-urea (46-0-0).