Nitrogen fertilization of forage sorghum

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Forage sorghum is important summer annual forage for livestock in Kansas. Many farmers are planting sorghum “feed” this summer due to concerns over a potential short hay crop. A number of questions have come in regarding N fertilizer, both yield response (in light of high N costs), and the potential for high nitrates if the summer remains dry.

Current N recommendations for sudangrass, forage sorghum, and millets in Kansas are to apply 40 to 50 pounds of N per acre at planting and again after each cutting, where multiple cuttings are obtained with full-season cropping. 

To re-test the validity of these recommendations, a study was initiated last summer near Randolph in Riley County. This sorghum was a brown mid-rib (BMR) hybrid, planted in mid-June. The conditions were generally good from planting until harvest July 20, with adequate, but not excessive moisture.

The yields, protein content, and nitrate concentration in the first cutting are reported in Table 1. Harvest was made at early boot stage, prior to heading. This resulted in slightly reduced yields, but very good quality forage, with protein levels exceeding 10% at recommended N rates. Nitrate levels in the forage were extremely high where the higher N rates – higher than recommended -- were applied, rendering the forage potentially toxic to cattle (>6,000 ppm nitrate is considered toxic). Thus this study confirms that N rates must be limited at planting. Applying all the N required for a full-season, multiple-cutting crop at planting is not a safe practice.

Table 1. Impact of increasing N rates at planting time on yield and quality of first cutting of forage sorghum

N rate, applied as UAN at planting time, coulter banded

Yield, first cutting (tons dry matter/acre)

Protein content, %

Nitrate in forage (ppm)

None

0.84

7.88

51

25 lbs N/a

0.98

7.69

296

50 lbs N/a

1.06

10.25

1,620

75 lbs N/a

0.94

10.25

8,600

100 lbs N/a

0.94

11.06

10,600

A number of N sources and methods of application were also compared. The results from these treatments on first cutting yields, protein content, and nitrate content are given in Table 2. All treatments were compared at the rate of 50 pounds per acre. Treatments were applied at planting time.

Table 2. Effect of N sources and application methods on yield and forage quality from first cutting

N source and application method (50 lbs N/acre)

Yield, first cutting (tons dry matter/acre)

Protein content, %

Nitrate in forage (ppm)

UAN, broadcast

1.01

9.63

2,540

UAN, surface banded

1.10

9.50

1,350

UAN, coulter banded

1.06

10.25

1,620

Urea, broadcast

1.07

10.00

4,960

All the products tested gave similar yields, protein levels, and nitrate contents. UAN surface banded or coulter banded, or urea broadcast, resulted in slightly higher yields than UAN broadcast. All the nitrate levels were found to be below the 6,000 ppm toxic level, though the broadcast urea was approaching that level of concern.

The data in Table 3 gives the yield and quality values for the second cutting of forage taken on September 2, 2011 from selected N treatments where 50 pounds of N was applied at planting and an additional 25 or 50 pounds was applied immediately after the first cutting to enhance yield and quality of the second cutting. Data reported is for the second cutting only. First cutting data is similar to that shown in Table 1 for the 50 pound N treatment.

Table 3. Effect of N rates, sources, and application methods on yield and forage quality from second cutting, made after heading

N rate, source, and method of application

Yield, second cutting (tons dry matter/acre)

Protein content, %

Nitrate in forage (ppm)

None

0.73

6.06

18

UAN, 25 lbs N/a surface band

1.17

5.69

21

UAN, 50 lbs N/a surface band

1.46

6.50

96

Urea, 25 lbs N/a broadcast

1.22

6.63

41

Urea, 50 lbs N/a broadcast

1.47

7.06

215

The data from the second cutting shows a significant response to N rate, but no difference among N sources. There was a significant response to the use of a urease inhibitor with urea at this time (data not shown). Moist soils and high temperatures in late July, when the application was made, created conditions conducive for ammonia volatilization from surface applications of urea. Also, this harvest was not made until after heading. This resulted in lower quality forage, as reflected by reduced protein content, but higher yields.

Summary

Current recommendations for N fertilizer rates for forage sorghum are 40-50 pounds of N per acre per cutting. This experiment confirmed that this is an appropriate rate to apply for optimum yield and quality. Exceeding these rates, even under good growing conditions, could lead to high nitrates in the forage, and be potentially toxic. Producers should not use rates above 50 pounds N per acre at planting in an attempt to save trips, and avoid the need to fertilize for a second cutting.

This experiment was conducted following a soybean crop, which may explain the relatively low response to N at the first cutting. Forage sorghum planted into corn stalks or doublecropped following wheat would likely respond to the recommended 40-50 pound N rate.

Concerns about N fertilizer effects on nitrate levels in the forage are important. Testing sorghum, sudan, or millet hay for nitrates is an important practice to ensure the safety of the cattle being fed. The K-State Soil Testing Laboratory offers nitrate analysis services to Kansas farmers and ranchers. The lab can be contacted at soiltesting@ksu.edu, or by calling 785-532-7897, for information on nitrate testing.


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