G.E. Selk , D.G. Wagner2, G.L. Strickland3,
S.E. Hawkins4, and S.Janloo5
Story in Brief
The effect of five different nitrogen fertilizer rates on nitrate accumulation
in hybrid sudangrass hay was examined in a two year study. A common
variety of Sudan X Sudan hybrid grass was grown in eastern and south central
Oklahoma over two consecutive summers. The hay was grown after applying
0 lb actual nitrogen per acre, 2 or 3 split applications of 50 lb nitrogen
per acre, 100 lb of nitrogen per acre, 150 lb nitrogen per acre, or 200
lb of nitrogen per acre. Hay samples showed significantly less nitrate
in those that had 0 lb of nitrogen. Samples from plots that received
200 lb of nitrogen had significantly greater concentrations of nitrate
than other plots. No difference between those plots with split applications
or equal amounts of 100 or 150 lb of nitrogen in a single application was
found. Year to year, cutting to cutting, as well as location variation
in nitrate concentration was quite large.
Introduction
Nitrate accumulation in sorghum forage may cause a toxicity and occasionally
death to ruminant livestock that consume high nitrate hays. Nitrogen
fertilizers are often used to increase forage yields when these types of
hay crops are grown. The additional nitrogen available from the fertilizer
may also enhance the accumulation of the potentially dangerous nitrate
in the hay. However, very little research has been conducted to quantify
the amount of nitrate accumulation that is directly related to nitrogen
fertilizer usage under Oklahoma growing conditions. Therefore, plots
of a common variety of sudangrass were grown after differing levels and
timing of nitrogen fertilization. This study allowed for the comparison
of different levels of nitrogen fertilization as well as the comparison
of two split applications of nitrogen versus a single application of nitrogen.
Materials and Methods
Varying levels of nitrogen fertilization for hybrid sudangrass were studied
at two Oklahoma State University Agronomy Research Stations; one located
at the Eastern Research Station near Haskell and another in the South-Central
Research Station near Chickasha. The Eastern Research Station plots
were planted on a Taloka silt loam soil. Whereas the South Central
Research Station is located on a Reinach silt loam. The nitrogen
fertilization rate study was conducted in two successive years. Three
cuttings of hay were taken at each location in the first year. Three
cuttings were obtained from the Eastern Research Station in year two; whereas,
only two cuttings were available from the South Central Research Station
in year two. Five fertilization treatments were replicated four times
at each of the locations each year. Therefore a total of 220 samples
were obtained. A randomized complete block design was used at each
location. At time of planting both soil and moisture conditions were
good at both locations for germination and emergence. The five levels
of nitrogen fertilization studied were:
Treatment 1: No nitrogen fertilization applied
Treatment 2: Two or three split applications of 50 pounds of actual
nitrogen per acre (50 pounds at planting time and 50 pounds per acre after
each cutting)
Treatment 3: One application of 100 pounds of actual nitrogen per acre
at planting
Treatment 4: One application of 150 pounds of actual nitrogen per acre
at planting
Treatment 5: One application of 200 pounds of actual nitrogen per acre
at planting.
The hybrid sudangrass variety utilized at both locations and both years
was a "Sudan X Sudan" hybrid named Monarch V* . Planting date in
year 1 was in late May for both locations. In year 2, the original
planting in both Eastern Oklahoma and South Central Oklahoma was late May.
However that planting was washed away at the Eastern Research Station and
replanting took place in late June. Each location was cut prior to
seed head exertion and plots in treatment 2 were top dressed to provide
the additional nitrogen. The hays were harvested with the cutter
bar height at 4 inches above the ground. Hay was harvested from each
of the 15 feet by 9 feet plots, and an approximate 1 pound sample consisting
of stems and leaves was labeled and taken to Stillwater for dry matter
and nitrate concentration determination. Nitrate concentrations were
determined by the salicyclic acid method described by Cataldo in 1975.
Nitrate concentration for each sample was expressed as parts per million
and adjusted to a 100% dry matter basis.
Data were analyzed by utilizing analysis of variance to test for main
effects of treatment, year, location, and cutting. All possible two-way
interactions of main effects were examined and then removed from the regression
model if non-significant. Least squares treatment means were compared
by examining "protected least significant differences".
Results and Discussion
The mean nitrate concentration across all treatments for each location
and cutting from both years is presented in Table 1.
Year, location, cutting, and fertilizer treatment were all significant
sources of variation for nitrate content of the samples. (P < .05)
There were no significant two-way interactions among treatments and the
other main effects. However, significant (P < .05) interactions
between year and location, year and cutting, and location and cutting were
present. These interactions could be attributed to weather stresses
that occurred in each of the two years, but at different times and to differing
degrees of severity at the two locations. Table 2 illustrates the
monthly rainfall for the two locations each year.
Drought stressed sudangrass plants have been shown to be nitrate accumulators.
The lack of rainfall at the Eastern Oklahoma Research Station in the summer
months of the second year of the study resulted in greater nitrate concentrations
in the first cutting of the second year.
Because there were no significant interactions of treatment with year,
location, or cutting, least squares means of the treatments across both
years, both locations, and all cuttings are presented in Table 3.
This data clearly illustrates that nitrogen fertilization affects nitrate
accumulation in hybrid sudangrass hays. Treatment 2 ( 50 pounds nitrogen
applied in two or three split applications) versus treatment 3 (100 lb
of nitrogen applied at planting) or treatment 4 (150 lb of nitrogen applied
at planting) revealed no signifcant difference in nitrate accumulation.
Therefore the practice of splitting nitrogen applications in order to reduce
nitrate toxicity appears to be questionable. Applying 200 lb of nitrogen
per acre resulted in increased concentrations of nitrate compared
to all treatments except where 150 lb were applied.
Nitrogen fertilization increased nitrate content of hybrid sudangrass
hays grown under differing rainfall amounts and in different locations
of the state. As large as these increases due to fertilization were,
they were still not as large as the differences noted between cuttings.
The differences due to cuttings were unquestionably related to weather
differences wherein hot, dry weather caused plant stress that resulted
in even greater changes in nitrate accumulation. Splitting the application
of nitrogen did not show a significant decrease in nitrate accumulation
in the sudangrass hays tested in this study.
Literature Cited
Cataldo, D. A., et al., 1975. Rapid colorimetric determination of nitrate
in plant tissue by nitration of salicylic acid. Commun. Soil Science
and Plant Analysis, 6(1), 71-80.
Table 1. Least squares means of nitrate concentrations (ppm) across
all treatments for each research station and cutting in 1990 and 1991.
Location
Eastern
South Central
Year 1
Cutting 1
5144
3685
Cutting 2
5301
4978
Cutting 3
5547
5330
Year 2
Cutting 1
21382
4210
Cutting 2
1427
6204
Cutting 3
8954
N/A
Table 2. Monthly rainfall totals for Eastern Oklahoma Research
Station and South Central Oklahoma Research Station in 1990 (year 1) and
1991 (year 2).
Eastern Research Station
Jan. Feb. Mar.
Apr. May Jun. Jul.
Aug. Sep. Oct.
Year 1 4.07 3.09
5.44 7.64 7.38
0.86 2.65 1.42 8.30
1.77
Year 2 0.81 0.06
1.10 2.36 6.56
3.51 0.97 0.51 5.15
4.59
South Central Research Station
Year 1 1.92 5.00
6.42 5.21 5.59
1.92 2.47 3.48 2.76
1.90
Year 2 1.49 0.05
1.51 3.28 6.72
3.80 3.41 3.75 9.88
3.47
Table 3. Least squares means of nitrate concentrations (ppm)
of hybrid sudangrass grown under different nitrogen fertilization schemes.
Treatment
0 lb. N
50 lb. 2-3 X
100 lb
150 lb
200 lb
3631a
6282b
6098b
7083bc
8432c
a,b,c Means with different superscripts are different
(P <.05).
