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Superintendent
David Davis
21262 Genoa Road
Linneus, MO 64653
Phone: 660 895-5121
FAX: 660 895=5122
Email: DavisDK@missouri.edu

Forage Systems Update
Vol 16, No. 3

About half of Missouri's 12 million acres of tall fescue receive nitrogen (N) fertilizer either in the spring or late summer to increase yields. Because ammonium nitrate production is being phased out and urea has ammonia volatilization problems, growers need comparative information on new N products for pastures.

Our primary objective was to compare ammonium sulfate, ammonium nitrate, urea, ESN coated urea, Nurea, and Nurea with 10% polymer coating, and mixtures of ammonium sulfate with urea and ESN as N sources for tall fescue in spring and late-summer applications. Secondary objectives were to determine if these sources of N changed the nutritive value of the forage or represent environmental risks due to nitrate leaching.

In 2006, plots established at the Bradford Research and Extension Center replaced those at the Forage Systems Research Center (FSRC). Another year of data was collected at the Southwest Missouri Research and Education Center (SWC). Each of the fertilizer sources and mixtures listed in Table 1 were applied in mid-March (spring, Experiment 1) and mid-August (late-summer, Experiment 2) in separate experiments at each location. The nitrogen fertilizer application rate was 75 lb/acre N. For the spring application, forage was harvested in late May, late July, and mid-October to measure season-long pasture production. For the late-summer application, plots were harvested in early December and indicate the suitability of each source for growing "stockpiled" forage for winter grazing.

Preliminary Results:
Experiment 1

Forage Yield.
Our preliminary data for the first two years indicate that only the initial harvest responded to N applied in March. Nearly 80% of the annual dry matter was harvested at the first sampling date in May and few treatment differences were measured in the two subsequent harvests; thus yields are reported as annual totals for both years (Tables 2). We hypothesized that the "coated urea" products might have yielded greater the summer or autumn after application because of their slow N release activity. But this was not the case. Additionally, no one product was overwhelmingly consistent in producing high yields.

We noted that ammonium sulfate ranked in the top producing group at nearly all harvests and locations and its performance is perhaps the most surprising data from this experiment. Another somewhat surprising result was that ammonium nitrate, urea, and ammonium sulfate proved to be nearly equal N fertilizer sources for tall fescue in spring. We should note that in each year both locations received ample moisture within 5 days of the fertilizer application to get urea into the soil solution. An extended dry period after application of these products may have resulted in more volatilization of urea and thus a comparative advantage for the "coated urea" products.

Thus far, our preliminary data show that a spring application of 75 lb/acre N increased yields by approximately 2250 lb/acre over the unfertilized control or about 30 lb of additional forage for each pound of N fertilizer applied. Ground moisture affected this relationship drastically as the range was 987 to over 4800 lb/acre.

Oil Analysis.
One of the questions posed while designing this research was whether the array of products and mixtures of products would alter the soil sulfate and/or nitrate such that forage quality might be enhanced. Another question regarded the fate of nitrate in the soil from different sources of N. At this point, we have one year of data for two locations (Table 3). When ammonium sulfate was the only source of N applied, the concentration of sulfate in the soil solution was approximately two and three times greater, respectively, than those treatments that had mixtures with ammonium sulfate and those that contained no ammonium sulfate. Nearly all plots treated with ammonium sulfate showed a decline in soil sulfate concentration during the growing season. Soil sulfate concentrations from plots treated with only ammonium sulfate declined nearly 35%, those with ammonium sulfate mixed with another product declined nearly 25%, while there was virtually no decline in soil sulfate where no sulfate was applied. It is yet to be seen if sulfate levels in forage increased as a result of the greater plant available sulfur in the soil solution. We are monitoring soil pH and in the final report will describe how it is affected by yearly applications of ammonium sulfate.

Soil nitrate concentrations were rarely influenced by the source of N applied to the plots. However on 19 May 2006, treatments that included polymer coated urea showed 4.8 ppm nitrate, which was nearly 40% greater than treatments without polymer coated urea. It is interesting that this slow release activity of the polymer did not occur at the other location. The soil nitrate concentration averaged across locations and treatments was 2.9 ppm (data not shown).

Forage Quality.
Only samples collected in Mt. Vernon from 2005 have been analyzed for nutrient content. Averaged over the three harvests, in vitro true digestibility of tall fescue was equal for nearly all treatments and averaged 69.8% when weighted based on yield. For crude protein, plots fertilized with ESN, ammonium sulfate, and mixtures of ESN and ammonium sulfate had about 1.0 percentage unit more crude protein at the first harvest (data not shown) than plots fertilized with other N sources. Averaged over the three harvests and all treatments, crude protein was 9.5%.

Experiment 2

Forage Yield.
For N applied in late-summer, many of the products yielded similarly and in most cases 10 or more of the products showed equal yields (Table 4). Consistently, urea, ammonium nitrate, and ammonium sulfate had comparable yields in three of four site-years. Tall fescue fertilized with urea yielded 35% less than that fertilized with ammonium nitrate during the dry autumn of 2005 in Mt. Vernon. Five days elapsed before any precipitation fell at the site and for 14 days only 0.20 inches of rain fell. This is a classic example of the risk associated with using urea as the N source for late-summer applications to pasture. Despite the promise in utilization of polymer coated urea to lessen this risk, it yielded less than most other treatments. The polymer coated urea has not shown much promise as a substitute for urea or ammonium nitrate for spring or late-summer N applications. We have yet to analyze the forage quality or soil fertility of samples collected in the autumn.

Table 1. Nitrogen fertilization treatments tested at the Southwest Research and Education Center near Mount Vernon, MO, the Forage Systems Research Center near Linneus, MO, and the Bradford Research and Extension Center near Columbia, MO. Each source is applied to deliver 75 lb/acre N. In addition, rate mixtures of ammonium sulfate/ESN, ammonium sulfate/urea and urea/ammonium sulfate/ESN are included.

Table 2. Total annual forage yield of tall fescue fertilized with different nitrogen sources at the Southwest Research and Education Center near Mount Vernon, MO and the Bradford Research and Extension Center near Columbia, MO. Nitrogen was applied 16 March ± 2 days in both years at 75 lb/acre for each fertilizer source. Values are the total of three harvests taken during the growing season.

Table 3. Soil sulfate concentrations at the Southwest Research and Education Center near Mount Vernon, MO and the Bradford Research and Extension Center near Columbia, MO. Nitrogen was applied 16 March ± 2 days at 75 lb/acre for each fertilizer source. Soil samples were collected to a six-inch depth on the dates reported below.

Table 4. Yield of autumn accumulated tall fescue fertilized with different nitrogen sources at the Southwest Research and Education Center near Mount Vernon, MO, the Forage Systems Research Center near Linneus, MO, and the Bradford Research and Extension Center near Columbia, MO. Nitrogen was applied August 17 ± 1 day in both years at 75 lb/acre for each fertilizer source. Forage was allowed to grow until harvested on the date reported below.

* Highest numerical yield within a column.
Bolded values within a column do not differ from the highest numerical yield.