Development of Forage Sorghum Tissue Testing
for Efficient Fertilization, 2010
Michael Ottman and James Walworth
Summary
A nitrogen fertilizer study was conducted in order to develop tissue testing guidelines for fertilizer
application to forage sorghum. The study was conducted at the University of Arizona Maricopa
Agricultural center on a sandy clay loam soil irrigated using furrow irrigation. Forage sorghum
was planted on 9 July 10 and fertilized with eight N rates varying from 0 to 350 lbs N/acre in 50 lb
N/acre increments. Maximum yield at final harvest was obtained at 100 lbs N/acre. The plants
were sampled five times during the growing season and the lower stem, most recently developed
leaf, and whole plant were analyzed for total nitrogen content. The stem tissue was also analyzed
for nitrate-N. The tissue test that was most sensitive to N fertilizer application and plant N status
was lower stem nitrate-N. Leaf and plant N levels increased somewhat with N fertilization
application, especially later in the growing season, but not nearly to the extent of lower stem
nitrate-N. Stem nitrate-N levels related to the optimum fertilization rate (100 lbs N/a) were 4000
ppm at the 14-leaf stage, 2000 ppm at the 18-leaf stage, and 1000 ppm at boot. Corresponding
values of stem nitrate-N for the previous year were 4500 ppm at the 13-leaf stage, 3000 ppm at the
18-leaf stage, and 2000 ppm at pre-boot, showing some consistency between years.
Introduction
The cost of nitrogen fertilizer is important to forage sorghum growers in irrigated areas. Sorghum silage may
require about half the nitrogen fertilizer as corn silage. So, when nitrogen fertilizer costs are high, sorghum has a
definite advantage over corn. Unfortunately, plant tissue testing guidelines for nitrogen fertilization have not been
developed for forage sorghum production in the desert southwest.
Soil and plant tissue guidelines for fertilizer application have been published for many crops grown in Arizona
(Doerge et al., 1991), but not for forage sorghum. The guidelines involve using a pre-plant soil test in combination
with post-plant tissue testing for nitrate. The pre-plant soil testing guidelines will not be addressed in this study but
the potential use of post-plant tissue testing will be investigated. The advantage of tissue testing guidelines is that
the plant can be an accurate indicator of its own nitrogen fertility status, and take into account growth rate and
sources of nitrogen in the soil and water. Tissue testing guidelines have been developed for wheat (Knowles et al.,
1991) and other crops where deficient, optimum, and excessive levels are determined and nitrogen fertilizer rates
developed for various growth stages. The study reported in this document was also conducted
The objective of this study is to develop post-plant nitrogen fertilization guidelines for forage sorghum for silage
based on nitrogen content in the plant. The results presented in this report are the second year of this study. The
results of the first year have been reported (Ottman and Walworth, 2010).
Materials and Methods
A forage sorghum nitrogen fertilizer experiment was established at the Maricopa Agricultural Center in Maricopa,
AZ on a Casa Grande sandy clay loam soil. The previous crop was barley. The nitrogen content in the top 6 inches
of the soil before planting was 5.5 ppm NO3-N and 14.3 ppm NH4-N, and the control plots 3 weeks after planting
averaged 3.4 ppm NO3-N and 5.1 ppm NH4-N in the top 5 ft of soil. The nitrate content of the irrigation water
averaged 1.9 ppm NO3-N. Atrazine was applied preplant for weed control. The forage sorghum hybrid Richardson
Seeds ‘Silo 700D’ was planted on beds 8 July 2010 at a seeding rate of 96,000 seeds per acre in 40 inch row
spacing. A germination irrigation was applied on 9 July 2010 and irrigation water was applied in furrows as needed
during the season (Table 2). Baythroid XL was applied on 4 August 2010 at 2.8 oz/acre to control various worms.
2010 Forage & Grain Report, College of Agriculture and Life Sciences, University of Arizona
73
Nitrogen fertilizer in the form of urea (46-0-0) was applied to plots 20 ft (6 rows) wide and 40 ft long. The seasonal
nitrogen rates were 0, 50, 100, 150, 200, 250, 300, and 350 lbs N/acre applied at four different times to achieve these
total rates (Table 2). The experimental design was a randomized complete block with 4 blocks.
Twenty plants per plot were sampled five times during the season for tissue testing and phytomass in an effort to
develop nitrogen fertilizer guidelines. Nitrate concentration was measured on the lower stems and total N was
measured on the newest fully developed leaf (showing a collar) and on the total plant. The portion of the lower stem
sampled was the crown before stem extension and the lower 4 inches of the stem after stem extension. The crown
tissue sampled was about ¼ to ½ inches in length and had minimal leaf and root tissue attached.
The center 5 feet of 4 rows in the plots were hand harvested at the soft dough stage on 30 Oct 2010 to obtain forage
yield. The plants were sampled for moisture determination and the yield was adjusted to 70% moisture content.
The data was statistically analyzed using the GLM and REG procedure in SAS.
Results and Discussion
Maximum forage yield was achieved at a nitrogen rate of 100 lbs N/acre in our study, and higher nitrogen rates
resulted in a yield plateau or slight yield decrease (Table 3). The optimum nitrogen rate under commercial
conditions may be different than that in this study depending on the nitrogen contained in the irrigation water and
soil, and the efficiency of N application. If nitrogen is applied in the irrigation water or applied in fewer than four
split applications, then the optimum N rate may be higher than that in our study. Forage moisture at harvest
increased with nitrogen rate and also achieved a maximum and leveled off at a nitrogen rate of 100 lbs N/acre. Stem
density at harvest was not affected by N rate. The plants had no productive tillers and any tillers present were very
small and insignificant. Nitrogen rate did not affect bloom date.
Increasing nitrogen fertilizer rates increased nitrogen uptake and the nitrogen content of various plant tissues
sampled on six dates throughout the growing season, but forage yield was not affected until the final harvest on 30
October (Table 4, Fig. 1). On the first sampling date (3 Aug, 7 leaf stage) none of the N tissue measurements were
affected by N application (Table 4). At the 10 leaf stage (14 Aug) stem nitrate-N and total N both increased with
increasing N application rate. Thereafter, all N measurements (leaf N, whole plant N, stem nitrate-N , and stem N)
were related to the amount of applied N. At the time of final harvest, yield increased as the N application rate
increased to 100 lbs/a, but additional N did not affect yield.
Plant tissue N levels, including leaf N, whole plant N, stem N, and stem nitrate-N were not strongly related to plant
growth, as illustrated by the very low correlation coefficients (r 2 values) shown in Table 1. Only in the 8/14
sampling did any r2 value exceed 0.5, indicating a very weak growth response to N fertilization.
Date
Leaf Nitrogen (%)
Plant Nitrogen (%)
Stem Nitrogen (%)
Stem NO3-N (ppm)
8/3/10
0.0002
0.1871
0.1010
0.0463
8/14/10
0.3230
0.6114
0.4004
0.0234
8/28/10
0.1075
0.1187
0.0271
0.0082
9/15/10
0.0041
0.0032
0.0034
0.0026
9/29/10
0.0888
0.0043
0.0010
0.0106
11/11/10
-0.0443
--Table 1. Correlation coefficients relating plant N parameters with plant fresh weight (at 70% moisture) throughout
the 2010 growing season.
2010 Forage & Grain Report, College of Agriculture and Life Sciences, University of Arizona
74
Plant weight (T/a @ 70% moisture)
25
20
3-Aug
15
14-Aug
28-Aug
10
15-Sep
29-Sep
5
30-Oct
0
0
100
200
300
400
Applied N (lbs/a)
Fig. 1. Applied N effect on plant dry weight measured at various times during the 2010 season.
The weak relationship between N application level and plant growth during the growing season is also evident in
Figure 1. From planting through the end of September (boot stage) N application level had very little effect on plant
growth. At final harvest, elevating the N application rate from 0 to 50 to 100 lbs/a increased the yield from 19.1 to
21.2 to 23.6 T/a, respectively. Addition of more N had no effect on plant growth or final yield. Plant growth
throughout the growing season is shown in Figure 2. The period of rapid growth occurred in August and September,
with growth slowing considerably later in the season.
30.000
Plant Dry Weight (T/a)
25.000
20.000
15.000
10.000
5.000
0.000
7/21
8/10
8/30
9/19
10/9
10/29
11/18
Date
Fig. 2. Plant dry weight accumulation over time for all the N application rates.
Stem nitrate-N, the plant N parameter identified as most sensitive to N status in 2009, declined over the growing
season similarly in 2009 (Figure 3) and in 2010 (Figure 4). The optimum stem nitrate-N can be identified as the
level associated with the largest final season yield (150 lbs N/a in 2009, and 100 lbs N/a in 2010). In 2009, the stem
nitrate-N values of the highest yielding plants was 4500 ppm on 8/27, 3000 ppm on 9/10, and 2000 ppm on 9/24 (the
dashed line in Figure 3). The roughly corresponding values for 2010 were 4000, 2000, and 1000 (the dashed line in
Figure 4). Thus, although the N response was diminished and the average tissue N levels elevated in 2010 relative to
2009, the optimum stem nitrate-N levels remained relatively constant, indicating the value of this measurement for
2010 Forage & Grain Report, College of Agriculture and Life Sciences, University of Arizona
75
evaluating plant N status. Furthermore, this test was able to identify N deficient plants very early in the season, long
before plant growth was affected by the N deficiency, unlike leaf and plant N.
7000
Stem Nitrate-N (ppm)
6000
0
5000
50
4000
100
3000
150
200
2000
250
1000
0
8/13
300
350
8/27
9/10
9/24
10/8
10/22
11/5
Date
Fig. 3. Sorghum stem nitrate-N over time for the various N application rates (2009).
14000
Stem Nitrate-N (ppm)
12000
0
10000
50
8000
100
6000
150
200
4000
250
2000
0
7/30
300
350
8/13
8/27
9/10
9/24
Date
Fig. 4. Sorghum stem nitrate-N over time for the various N application rates (2010).
2010 Forage & Grain Report, College of Agriculture and Life Sciences, University of Arizona
76
Acknowledgments
This project was funded by the United Sorghum Checkoff Program. The technical assistance of Richard and Glenda
Simer and Mary Comeau is greatly appreciated.
The seed used in this project (Richardson Seeds ‘Silo 700D’)
was donated by Desert Sun Marketing Company, Inc.
References
Doerge, T. A., R. L. Roth, and B. R. Gardner. 1991. Nitrogen fertilizer management in Arizona. College of
Agriculture, Univ. of Arizona, Tucson.
Knowles, T. C., T. A. Doerge, and M. J. Ottman. 1991. Improved nitrogen management in irrigated durum wheat
production using stem nitrate analysis. II. Interpretation of basal stem nitrate-N concentrations. Agronomy Journal
83:353-356.
Ottman, M., and J. Walworth. 2010. Development of forage sorghum tissue testing for efficient fertilization.
Forage & Grain Report 2010. Univ. of Ariz. College of Ag. and Life Sciences, Tucson.
http://cals.arizona.edu/pubs/crops/az1526/az1526e.pdf.
2010 Forage & Grain Report, College of Agriculture and Life Sciences, University of Arizona
77
Table 2. Irrigation and fertilizer schedule for a forage sorghum fertilization experiment conducted at Maricopa, AZ
in 2010. A total of 3.7 inches of rainfall was recorded during the growing season.
Seasonal N rate (lbs N/acre)
Date
Stage
Irrigation
0
50
100
150
200
250
300
350
inches
---------------------------------------- lbs N/acre ---------------------------------------07/09
07/13
07/28
08/12
08/25
09/09
09/22
10/06
Sum
Planting
2-leaf
6-leaf
10-leaf
14- leaf
18- leaf
22-leaf
Bloom
2.65
2.63
1.62
2.74
2.83
3.34
2.98
2.74
21.53
0
--0
--0
--0
--0
12.5
--12.5
--12.5
--12.5
--50
25.0
--25.0
--25.0
--25.0
--100
37.5
--37.5
--37.5
--37.5
--150
50.0
--50.0
--50.0
--50.0
--200
62.5
--62.5
--62.5
--62.5
--250
75.0
--75.0
--75.0
--75.0
--300
87.5
--87.5
--87.5
--87.5
--350
Table 3. Nitrogen (N) rate effect on forage yield adjusted to 70% moisture, forage moisture, stem density at harvest,
date of 50% bloom, and plant height.
Plant
Forage
Forage
Stem
Bloom
Height
N Rate
Yield
Moisture
Density
Date
Inches
lbs N/A
T/A
%
Stems/A
0
50
100
150
200
250
300
350
Average
19.1
21.2
23.6
22.4
23.1
22.7
23.2
22.6
22.2
68.5
71.1
73.5
71.4
73.3
74.3
73.8
73.1
72.4
68,211
60,743
69,705
61,739
68,211
68,709
61,241
63,730
65,286
10/7
10/6
10/7
10/6
10/6
10/6
10/5
10/7
10/6
6.23
6.43
6.56
6.58
6.63
6.56
6.48
6.28
6.47
CV (%)
Linear
Quadratic
Cubic
8.7
*
*
NS
2.3
**
*
NS
7.9
NS
NS
NS
0.01
NS
NS
NS
5.6
NS
NS
NS
2010 Forage & Grain Report, College of Agriculture and Life Sciences, University of Arizona
78
Table 4. Nitrogen fertilizer rate effect on forage yield adjusted to 70% moisture, nitrogen uptake, stem nitrate and
total N, leaf nitrogen on a percentage and leaf area basis, and plant nitrogen at various growth stages.
Forage
Nitrogen
Stem
Stem
Leaf
Plant
Growth
N Rate
Yield
Uptake
Nitrate
Nitrogen Nitrogen Nitrogen
Date
Stage
lbs N/a
T/A
lbs/A
ppm NO3-N
%
%
%
08/03
08/14
08/28
7-leaf
10-leaf
14-leaf
0
50
100
150
200
250
300
350
0.19
0.20
0.18
0.26
0.19
0.19
0.20
0.16
4.75
4.94
4.58
7.15
4.94
4.98
5.25
4.17
9,144
10,817
11,359
11,336
12,093
11,358
12,110
12,280
5.07
5.53
5.75
5.22
5.47
5.85
5.51
5.53
4.44
4.39
4.56
4.43
4.55
4.59
4.60
4.47
4.00
3.85
4.09
3.96
4.20
4.08
4.23
4.21
Average
CV (%)
Linear
Quadratic
Cubic
0.20
5.03
11,311
5.50
4.50
4.08
35.1
ns
ns
ns
29.9
**
ns
ns
12.6
ns
ns
ns
6.0
ns
ns
ns
3.9
ns
ns
ns
6.9
+
ns
ns
0
50
100
150
200
250
300
350
2.22
2.13
2.21
2.33
2.15
2.19
2.53
2.13
41.2
41.8
44.9
45.3
43.9
43.7
49.5
44.2
5,748
8,293
9,607
9,497
10,261
10,058
10,644
10,327
3.08
3.66
3.45
3.61
3.69
3.60
3.50
3.83
3.69
3.75
3.76
3.73
3.80
3.64
3.75
3.74
3.06
3.21
3.38
3.27
3.33
3.27
3.20
3.40
Average
CV (%)
Linear
Quadratic
Cubic
2.24
44.4
9,419
3.57
3.74
3.27
18.0
ns
ns
ns
14.5
ns
ns
ns
17.5
**
*
ns
9.0
*
ns
ns
4.1
ns
ns
ns
6.4
ns
ns
ns
0
50
100
150
200
250
300
350
6.66
5.88
6.02
6.16
6.00
6.32
6.37
6.08
78.1
80.9
86.8
89.0
88.6
96.9
95.9
95.7
1,198
1,530
3,214
4,656
6,197
6,774
5,346
6,447
1.38
1.67
1.93
2.00
2.13
2.25
2.12
2.34
2.31
2.65
2.74
2.72
2.83
2.86
2.83
3.02
1.98
2.37
2.47
2.46
2.49
2.58
2.53
2.64
Average
CV (%)
Linear
Quadratic
Cubic
6.17
89.3
4,524
2.00
2.76
2.45
14.8
ns
ns
ns
13.9
*
ns
ns
33.0
**
*
ns
10.2
**
*
ns
6.6
*
ns
+
6.0
**
*
*
2010 Forage & Grain Report, College of Agriculture and Life Sciences, University of Arizona
79
Table 4 (con’d). Nitrogen fertilizer rate effect on forage yield adjusted to 70% moisture, nitrogen uptake, stem
nitrate and total N, leaf nitrogen on a percentage and leaf area basis, and plant nitrogen at various growth stages.
Forage
Nitrogen
Stem
Stem
Leaf
Plant
Growth
N Rate
Yield
Uptake
Nitrate
Nitrogen Nitrogen Nitrogen
Date
Stage
lbs N/a
T/A
lbs/A
ppm NO3-N
%
%
%
09/15
09/29
10/30
18-leaf
Boot
Harvest
0
50
100
150
200
250
300
350
11.81
12.07
11.89
12.63
10.78
11.26
12.35
12.30
89
116
121
133
123
127
136
147
372
420
1,725
2,426
3,435
3,733
3,744
3,537
0.44
0.65
0.94
1.09
1.12
1.19
1.18
1.28
1.74
1.98
2.11
2.21
2.23
2.26
2.22
2.11
1.30
1.64
1.76
1.85
1.96
1.92
1.87
2.05
Average
CV (%)
Linear
Quadratic
Cubic
11.89
125
2,490
1.00
2.12
1.81
19.1
ns
ns
ns
20.3
**
ns
ns
24.6
**
**
*
10.7
**
**
ns
8.2
**
**
ns
8.6
**
*
+
0
50
100
150
200
250
300
350
19.08
20.45
20.75
19.34
19.64
20.38
18.65
19.71
119
157
187
176
179
192
175
194
557
214
621
1,238
1,401
2,063
1,841
1,866
0.35
0.38
0.53
0.62
0.76
0.76
0.76
0.83
1.94
2.40
2.57
2.53
2.63
2.56
2.57
2.57
1.05
1.32
1.52
1.58
1.54
1.59
1.59
1.67
Average
CV (%)
Linear
Quadratic
Cubic
19.77
174
1,247
0.63
2.49
1.50
12.3
ns
ns
ns
16.1
**
*
+
48.9
*
ns
+
15.9
**
*
ns
5.6
**
**
**
8.2
**
**
*
0
50
100
150
200
250
300
350
19.09
21.21
23.63
22.37
23.08
22.72
23.15
22.64
53.7
85.4
101.3
116.7
120.5
112.2
113.6
121.8
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
0.47
0.67
0.71
0.87
0.87
0.83
0.82
0.91
Average
CV (%)
Linear
Quadratic
Cubic
22.24
103.2
15.0
**
**
+
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
0.77
8.7
*
*
ns
2010 Forage & Grain Report, College of Agriculture and Life Sciences, University of Arizona
14.7
**
**
ns
80