Sudan is a very heavy feeder and good growth is dependent on nitrogen being available. Well fertilized sudangrass should be about the same deep green as well fertilized silage corn. If it is yellow due to nitrogen deficiency, yield is being sacrificed. Sudangrass planted in the late summer with a yield potential of about 12 tons needs between 125 and 150 pounds of actual nitrogen per cutting. If this is to be applied with the irrigation water, most of the nitrogen should be applied so that it is available as the plants are making rapid growth.
Sudangrass is very good at capturing nitrogen applied in pondwater. If pondwater is available, it is not only an excellent source of fertilizer for the sudangrass, but is also a good place to put pondwater so the pond can be as low as possible going into the winter.
For a rough guess at how much pondwater to apply, use the following method:
1. Take a sample of pondwater to the lab as close to the time of use as possible. It would be best to take a sample of pure pondwater as it is coming out of the pond, but this may not be possible. If sampling from the pond itself, obtain a container from the lab and attach the container to a pole. Sampling directly into the container avoids having the solids, which also contain some nitrogen, settle and remain in the dipping bucket, throwing off the analysis. Run the agitator before and during sample collection to help the pond be more uniform. Refrigerate the sample immediately and keep it cold until you deliver it to the lab.
Ask the lab for ammonium (NH4-N) and total N (TKN). Asking for nitrate (NO3-N) is not necessary. Very little of the nitrogen in the pondwater is in the nitrate form because nitrate is only formed in the presence of oxygen and anaerobic lagoons have no oxygen in them. Fertilizer or manure ammonia is converted to nitrate only after it is applied to the field.
2. Multiply ppm ammonium nitrogen (from your lab report) by 2.7 to get pounds of ammonia N in an acre foot of pond water. If your lab reported in percent, multiply percent N by 10,000 to get ppm.
.012% ammonia nitrogen x 10,000 = 120 ppm N
120 ppm N x 2.7 = 324 pounds of N in an acre foot of pond water
3. Measure your pond surface area, length times width. Divide by 43560 square feet per acre to get acre-feet of pond surface.
if your pond is 600' x 150': 600' x 150' = 90,000 sq ft.
90,000 sq ft ( 43560 sq ft per acre = 2.07 acres.
4. Multiply pounds of nitrogen in an acre foot of pond water by the number of acres in the pond to get pounds of nitrogen in a 1 foot drawdown of pondwater*.For example:
324 lbs N x 2.07 acres = 671 lbs ammonia nitrogen going out if you draw down the pond one foot.
If you are irrigating a 40 acre field and want to apply 75 units of nitrogen, you will need 75 x 40 or 3000 pounds of nitrogen for the whole field.
5. Divide the total nitrogen needed for the field by the amount of nitrogen in each foot of pond drawdown to get the total drawdown needed on the field.
3000 lbs N total needed ( 671 lbs N per foot of pond drawdown = 4.5 feet of pond drawdown on that 40 acre field to get 75 units of ammonia nitrogen per acre.
This method gives you what is probably a minimum application rate. Using the TKN figure will give you both the ammonium nitrogen and the nitrogen tied up in the solids, which will become available a little later. Also, it is likely that your pondwater will increase in nitrogen concentration between the time you take your sample and the time you actually irrigate.
This is admittedly a crude method of figuring nitrogen application, and would probably not be accurate enough to use on corn. However, it should be sufficient for sudan, and will help you figure if you have enough nutrients in your pondwater to avoid additional fertilizer purchases. We plan to test other methods which will make it easier to use pondwater as a nitrogen source.
Some growers are hesitant to apply enough nitrogen to sudangrass because of the danger of nitrate poisoning. In most cases, this can be avoided without having to short the crop on nitrogen. Conditions favoring toxic nitrate accumulation include:
1) excessive nitrogen fertilizer rates (the crop will be a very, very dark green),
2) cutting when less than 18 inches tall, and 3) cutting on cold, foggy mornings or immediately after a moderate frost.
If the crop is being harvested for silage, high nitrate levels should come down by about half during the ensiling process. The best way to avoid nitrate toxicity is to harvest suspect sudangrass for silage, then test after about two weeks in the pit. If the levels are still high, the silage may still be safely fed if it is blended with other appropriate feeds.
*Because the sides of a pond are sloped, the pond surface area decreases as the pond is drawn down. For a handy, customized chart of your pond capacity for each foot of drawdown, call our office and give us your pond length and width at maximum fullness, approximate pond depth, and your address or fax number.
November 5, 1999