Fertilization of Extremely Grazed and Moderately Grazed Mixed-Grass Prairie with Slow Release Phosphorus and Urea

J. M. Volk, W.T. Barker, P. Nyren and D. Whitted

Table of Contents

• Abstract
• Objectives
• Background
• Methods
• Results
• Production
• Species Diversity on the Extremely Grazed Pasture
• Species Diversity on the Moderately Grazed Pasture
• Plant Phosphorus Concentrations
• Conclusions

Abstract

Advantages of a balanced native pasture consisting of cool-season and warm-season grasses and a diverse forb mixture are an increase in forage quality and forage production, a longer grazing season, an increase water permeability and healthier soils. In a three year study on ways to achieve this balance, fertilization was followed by the measurement of plant production, plant phosphorus concentrations, plant available soil phosphorus and species diversity.

Response of extremely and moderately grazed mixed-grass prairie vegetation to annual applications of slow release phosphorus (P) and slow release urea (N) was studied during a three-year period at the Central Grasslands Research Extension Center near Streeter, North Dakota. Fertilizer rates of 0, 24, and 48 pounds/acre of elemental P were applied May 1 and June 20. Fertilizer rates of 33 and 66 pounds/acre of elemental N were also applied June 20.

Generally, changes due to fertilization over the three-year period were more prominent on the extremely grazed pasture than the moderate grazed pasture. Forb diversity significantly increased on the extremely grazed pasture while grass diversity had no change. The moderate grazed pasture showed significantly lower grass diversity on almost all treatments and forbs stayed virtually unchanged. Overall plant phosphorus concentrations had statistically insignificant increases, but the resulting increase of plant phosphorus content was enough to meet or exceed the minimum phosphorus requirements for a 1,200 lb lactating cow longer in the grazing season. No significant change occurred with total production on either grazing treatments.

Objectives

1. To determine the changes in species composition resulting from fertilization with slow release phosphorus (P) and urea (N) on extremely grazed and moderately grazed mixed-grass prairie.

2. To determine the production of cool-season grasses, warm-season grasses, forbs, and shrubs on the slow release fertilizer treatments.

3. To determine the seasonal uptake of phosphorus in selected forage species.

4. To determine the soil phosphorus levels in different range sites or landscape positions on the CGREC.

Background

Previously in North Dakota most range fertilization studies have involved using ammonium nitrate and super-phosphate fertilizers. These are fast release fertilizers. Cool-season grasses dominate Northern Great Plains Grasslands, and with fast release fertilizers cool-season species are usually favored and warm-season species usually decrease in the species composition. When grazing animals harvest the forage each year they remove nitrogen and phosphorus from the system. Over the long term, productivity and species composition of the grassland can be affected.

A well-planned fertilization program can increase grassland productivity through rapid range improvement, and help restore depleted ranges to near original productivity. Extended periods of overgrazing, early spring grazing and higher than recommended stocking rates create low condition rangelands. Low condition ranges have poor surface conditions, reduced nutrient level, increase in less desirable species, reduced total forage production, and imbalances in cool-season and warm-season native grasses (Hill 1987). Results from a six year study in North Dakota showed that two years of fertilization with 90 lbs N/acre on a heavily grazed pasture did more to improve range condition and production than did 6 years of complete isolation from grazing (Rogler and Lorenz 1957).

Substantial changes in native grassland communities due to fast release fertilization are well documented in the Northern Great Plains. The most immediate and easily measured response to nitrogen fertilization has been the general increase in forage production and improvement of forage quality (Goetz et al. 1978). A less well understood, but equally recognizable, response due to fertilization is the almost universal shift in plant species dominance from one of warm-season short grasses to cool-season mid-grasses (Goetz et al. 1978).

Methods

The field study was conducted at the Central Grasslands Research Extension Center located in Kidder and Stutsman counties, North Dakota. Two sites of 150 feet by 200 feet were chosen, one being in an extremely grazed pasture and the other in a moderate grazed pasture. The extremely grazed pasture is characterized by having 20 % of the forage produced in an average year remaining at the end of the year verses the moderate grazed pasture that has 50 % of the forage produced remaining at the end of the year. Within each site, four replicates of 70 feet by 50 feet were laid out and seven plots were placed within each replicate. Individual plot size is 10 feet by 50 feet.

The seven treatments were: control, 24 # P/acre applied May 1, 48# P/acre applied May 1, 24# P/acre applied June 20, 48# P/acre applied June 20, 33# N/acre applied June 20, 66# N/acre applied June 20. These seven treatments were randomly placed in each of the four replications on the two grazing intensities. Grazing pressure was removed by an electric three-wire fence. Fertilizer was applied using an ATV and a Gandy fertilizer spreader calibrated accordingly for the different rates.

Over the course of the summer four clippings were taken of five plant species to measure the uptake of phosphorous. Clipping dates occurred approximately between the fifth and seventh of June, July, August and September. The five species clipped were western wheatgrass (Agropyron smithii), green needle grass (Stipa viridula), Kentucky bluegrass (Poa pratensis), smooth brome (Bromus inermis), and western snowberry or buckbrush (Symphoricarpos occidentalis). Also a separate clipping was performed to establish above ground production on each of the treatments.

Results

Production

Production data were taken in the summers of 2000, 2001, and 2002. In the extremely grazed pasture (Table 1) the production of cool-season grasses (p=0.09), warm-season grasses (p=0.61), forbs (p=0.43), and shrubs (p=0.85) showed no effects due to treatment. In the moderately grazed pasture (Table 2 (p. 12)) cool-season grasses (p=0.45), warm-season grasses (p=0.61), forbs (p=0.23), and shrubs (p=0.18) also had no changes in production due to treatment. Production on the extremely grazed pasture was different by year on the cool-season grass (p=0.00) and forbs (p=0.00). The moderate grazed pasture had yearly differences in production for the cool season grass (p=0.00), forbs (p=0.00), and shrubs (p=0.00).

Species Diversity on the Extremely Grazed Pasture

Species diversity in the extremely grazed fertilization treatment showed no significant changes in grasses from 2000 to 2001 (Figure 1). Grasses were very similar from 2000 to 2001 with very little change for any of the fertilizer treatments. Forbs on the other hand, had significant increases in diversity in the 24# P May 1 and 48# P May 1 fertilizer treatments.

Figure 1. Shannon-Weiner Diversity Index applied to the results from the extremely grazed pasture in 2000 and 2001. Fertilizer treatments are listed on the X axis; Y axis is diversity index with 0 indicating low diversity and 4.5 high diversity.

Species Diversity on the Moderately Grazed Pasture

Species diversity on the moderately grazed pasture resulted in significant decreases in grasses from 2000 to 2001 on the control, 24# P May 1, and 48# May 1 fertilizer treatments (Figure 2). Literature cites grass diversity characteristically decreasing with nitrogen fertilization which we are seeing on the nitrogen treatments, but phosphorus treatments are also decreasing in grass diversity on this grazing treatment. Forb diversity from 2000 to 2001 only had a significant increase on the 33# N June 20 fertilizer treatment. The fact that the control also had decreases in grass diversity may indicate that something else besides the fertilization is causing the decrease in grass diversity. Possibly it could be the accumulation of excess litter, removal of grazing pressure, rain patterns, or other environmental factors.

Figure 2. Shannon-Weiner Diversity Index applied to the results from the moderate grazed pasture in 2000 and 2001. Fertilizer treatments are listed on the X axis; Y axis is diversity index with 0 indicating low diversity and 4.5 high diversity.

Plant Phosphorus Concentrations

Plant phosphorus concentrations are an important aspect for animal nutrition. Meeting the minimum cattle requirement longer in the grazing season can promote a longer grazing season and possibly cut back on some supplementation costs. The extremely grazed pasture seemed to have lower overall phosphorus concentrations and more treatments not meeting the minimum requirements. Results of the plant phosphorus concentrations (Table 3) showed definite trends of sporadic natural phosphorus occurrence. The control treatment and two nitrogen treatments did not meet the minimum requirements for most of the grazing season; however, the 24# P and 48# P treatments which consistently met the minimum phosphorus requirements. The 48# P June 20 seemed to have the best results on the extremely grazed pasture.

The plant phosphorus concentrations on the moderate grazed pasture showed more stable and consistent results (Table 4). Once again the control and two nitrogen treatments did not do well in meeting the minimum phosphorus requirements throughout the grazing season. The four phosphorus treatments did extremely well on the moderate grazed pasture meeting or exceeding the minimum requirements virtually on all clipping dates and plant species.

Minimum phosphorus requirements are for a 1,200 lb lactating cow. Bold print signifies plant phosphorus values that met or exceeded the minimum phosphorus requirement.

Conclusions

Having a well balanced pasture with warm-season grasses, cool-season grasses, and forbs will maximize economic returns for a rancher. With a balance of cool-season and warm-season grasses the nutritive requirements of a cow can be maximized throughout the grazing season. Phosphorus fertilization not only benefits the plants but improves plant nutrition longer into the grazing season.

• With additions of phosphorus we are seeing low phosphorus levels in plants begin to increase to meet the minimum cattle requirements later in the grazing season. This extra phosphorus will not only benefit the cattle to meet their minimum requirements but will also benefit the vegetation. More time is needed to determine the full effects of the change in plant species composition and diversity due to slow release fertilization.
• Initial results are showing trends that slow release nitrogen is acting in a manner similar to fast release nitrogen fertilizer, increasing cool-season grasses and production.
• More time is needed to better understand the species composition change from a slow release phosphorus fertilizer. Literature shows phosphorus fertilization can take three years or more to show the changes in species composition.
• The most important point is that fertilization can extend the length of time in the grazing season in which the plants are meeting the minimum phosphorus levels for cattle.