Members of the Forage and Nutrition Group (FANG) worked with the Curry County Soil & Water Conservation District to conduct a disciplined trial on well-managed dense grass pastures to determine the effects of various levels of nitrogen fertilizer on forage yield, the efficiency of nitrogen capture, forage nutritional value and the potential losses of nitrogen into groundwater. Five nitrogen treatments of 0, 100, 200, 400 and 800 units of N per acre per year, applied in four equal split-applications of urea (46-0-0-0) were replicated on three ranches. Non-irrigated experiment plots measured 3 x 20 feet and were planted in dense perennial ryegrass.
We managed the plots to duplicate good grazing practices through the typical cycle of grass growth in well-managed coastal Oregon grass pastures. We measured nitrogen loss into the groundwater by sampling the groundwater in wells dug in each plot and also as extractable nitrogen in soil samples taken from below the root zone in each plot.
Seven collections were made during the November-July growing season. Measurements included dry matter yields, nitrogen and other nutrients and also nitrogen in the groundwater and soil extracts.
Our primary objective was to investigate the effects of various levels of nitrogen fertilizer on the nitrogen use efficiency, forage quality and water quality in well-managed dense grass pastures on the Oregon coast. We wished to maximize the benefit received from nitrogen applications and also prevent leaching of excess nutrients into rivers and streams.
In autumn 2004, we fenced areas on three cooperating ranches, worked up the soil and planted these areas with an improved variety of perennial ryegrass. In October 2005, we laid out the 15 test plots and applied the treatment levels of nitrogen fertilizer. Through the growing season of 2005-2006, we applied our four split-applications of nitrogen fertilizer on October 3, January 23, April 13 and May 15. Fertilizer was applied by hand in a carefully controlled technique to minimize variation within a plot.
On each collection date, we carefully harvested forage in each plot by lawnmower to leave a residual height of 2 inches. Plots were not irrigated so all forage growth depended on natural rainfall, which typically begins in the autumn.
We sent dried samples from each plot to a commercial laboratory to test for nitrogen, which allowed us to calculate the nitrogen yields and other variables. We collected water samples on April 16, 2007, after a period of heavy rainfall.
We observed in this data that the nitrogen fertilizer increased the grass yield in the spring and early summer. Surprisingly, extra nitrogen had minimum effects on forage growth in the fall and early winter.
The baseline crude protein levels of this perennial ryegrass were extremely high—greater than 20% through the autumn and winter. Only during the summer did these levels drop below 16%, but even at these high baseline values of crude protein, we observed that any added nitrogen fertilizer still elevated these levels further. However, from an animal growth perspective, even the baseline crude protein level of this perennial ryegrass was generally higher than the nutritional needs of growing cattle and lambs. In the early summer, however, the extra nitrogen helped maintain the crude protein levels above the requirements for growing lambs, while the unfertilized control dropped below 16%.
From an economic perspective, our results show diminishing returns with higher nitrogen applications in this type of soil, and suggest that an economic cutoff point may be somewhere near 200 lb N/acre/year. From an environmental perspective, the ANR values indicate that higher application rates of nitrogen result in a considerably lower percentage of that nitrogen captured in the forage and a higher potential nitrogen loss into the soil and groundwater.
The soil nitrogen (nitrates and nitrites extracted from the soil) below the root zone clearly shows that low levels of nitrogen fertilizer did not increase the amount of residual nitrogen potentially available to groundwater loss. But nitrogen fertilizer applications of 400 lb/acre or higher did increase the levels of this deep soil extractable nitrogen.
Extra nitrogen at 100 and 200 lbs/acre rate gave a significant yield boost in the spring, and the higher levels of 400 and 800 lbs/acre gave an additional yield response but not double. These observations have important economic implications. Nitrogen fertilizer also boosted the crude protein levels in the forages, even when the controls had CP levels of greater than 20%. Nitrogen fertilizer had no effects on forage TDN, phosphorus, potassium, magnesium and sulfur, but it did seem to reduce forage calcium levels during the winter and spring.
Nitrogen fertilizer showed diminishing returns in terms of the efficiency of nitrogen capture and yield response. Our data suggest an economic cutoff point may be somewhere near 200 lb N/acre/year. Although tests on the well water were inconclusive, the tests of soil below the root zone showed that low levels of nitrogen fertilizer did not increase the amount of residual nitrogen potentially available to groundwater loss, but that nitrogen fertilizer applications of 400 lb. acre or higher did increase the levels of this deep soil extractable nitrogen.
1) Extra nitrogen improved yields of perennial ryegrass compared to the non-fertilized control, particularly during the spring and early summer.
2) The yield responses seemed to cluster in two groups: nitrogen applications of 100 and 200 lb/year and nitrogen applications of 400 and 800 lb/year.
3) The two higher N application rates did not double the forage yields over the two lower application rates.
4) Higher N application rates are less efficient than lower N application rates in respect to yield response and nitrogen retention.
5) Nitrogen applications had different effects at different times during the growing season.
6) Extra nitrogen increases crude protein levels of the grass, but this may be nutritionally important only near the end of the growing season.
7) Nitrogen fertilizer applied as urea has no effect on most major minerals but may suppress forage calcium levels in the winter and spring.
8) Nitrogen applications less than 400 lb N/acre/year do not seem to increase the amount of N leaching below the root zone. Application rates of 400 lb/acre/year or higher may increase the amount of extractable nitrogen below the root zone. Soil characteristics seem to affect this response.
We observed that typical application rates of nitrogen fertilizer may not result in excess nitrogen leaching into the groundwater from dense, well-managed grass pastures. Our observations support the sustainable environmental value of well-managed grass pastures. Our results, however, may be in contrast with observations by other researchers concerning nitrogen applications to row-crop farming situations.
Ranchers are actively discussing how to increase and maintain the density of their pastures. In addition, we are exploring strategies for differential rates of nitrogen applications based on time of year. These discussions are ongoing in rancher groups throughout western Oregon.
RECOMMENDATIONS OR NEW HYPOTHESES
This trial design should be duplicated in other places in the west-side Pacific Northwest to include different soil characteristics and different rainfall patterns. Our results were derived from a unique environment: specific soil types and rainfall patterns on the south coast of Oregon. With additional data from other sites, response curves and economic models can be formulated that have broader economic and environmental applications.
Additional deep soil tests (below the root zone) should be carried out at other sites, under different fertility and pasture situations, both for pastures and for row crops.
The technical advisor provided ongoing updates and data summaries throughout the trial period to ranchers in three producer study groups in western Oregon. These meetings have been conducted monthly, so that ranchers have been routinely apprised of the ongoing project. In addition, three public producer meetings have been conducted on the south coast and inland in which this project has been described. Educational activities are still ongoing. Additional public meetings are planned, and the final results and conclusions will be uploaded to the Curry Soil & Water Conservation District website and to the Forage Information System website of Oregon State University.