By Bob Hendershot, State Grassland Conservationist, USDA-NRCS-Ohio
Forages can help in managing nutrients from manure applications.
Harvested forages can be used effectively to remove excess nutrients from crop fields and reduce water pollution potential. Harvested forages are very marketable, and should be considered as a way to transfer nutrients off of farms with excess nutrient levels. Forages also are excellent in improving soil conditions, reducing soil erosion and runoff that contributes to water quality concerns.
Forages can be used to draw down soil test phosphorus levels in fields with excessive soil test levels. Typically,
forages will remove 13 to 15 pounds of P2O5 per ton of harvested forage. Plants harvested earlier in their growth stage will have a higher concentration of phosphorus, but a lower yield per acre. Different forages remove different
amounts of phosphorus. Oklahoma research shows orchardgrass removing 50% more P2O5 than the same yield of
alfalfa, ryegrass or tall fescue; twice as much as red clover and three times more than sorghum-sudangrass or pearl millet. Cornell research has ryelage with more P2O5 than oatlage, which has more than wheatlage.
The preferred nutrient management plan for soils with adequate soil test levels is to not apply more nutrients than the crops can use and to draw down nutrients on soils with very high and higher soil test levels. The critical nutrient in most cases is phosphorus.
Phosphorus is the least of the major nutrients removed by most cropping systems. Manures typically have more nitrogen and potassium than phosphorus. The Best Management Practice is to apply manure to meet the crops’ phosphorus requirement then supplement the additional nitrogen and potassium from other sources that do not contain phosphorus. If manure is applied to meet the nitrogen requirement of the crop then excessive amounts of phosphorus accumulate in the soil or are lost into the environment.
Soil tests are recommended every three years on fields receiving manure to help farmers monitor these soil nutrient levels. It is a slow process to both build and to reduce soil test values.
Generally, it takes 10 to 20 pounds of P2O5 per acre to raise the soil test level by 1 pound per acre. The same is true to reduce the soil test level by 1 pound per acre; 10 to 20 pounds of P2O5 per acre will need to be removed. It is a very slow process: harvesting 5 tons of hay per acre for 20 years from an excessively high soil test phosphorus field not receiving any additional phosphorus would drawdown the soil test value only about 100 pounds per acre.
The forage needs to be harvested and removed from the field to reduce the nutrient level in the field. Grazing systems do not effectively remove nutrients from the field since most of the nutrients are recycled back to the land by the animals during the grazing event. The animals will deposit the nutrients near watering points and loafing areas in fields that are continuously grazed. An improved rotational grazing system will have a more uniform distribution of the nutrients by the animals.
Harvested forages do not just need to come from hay fields. Cover crops harvested as forages can help manage
nutrients on livestock farms. Conservation planners for many years have recommended the use of cover crops after corn silage and soybeans to reduce soil erosion. This Best Management Practice has many other benefits including forage production, nutrient management, soil health, and weed and disease management.
Cereal crops like rye, wheat, triticale, barley and oats used as a cover crop can produce substantial forage dry matter yields. They can produce 2 to 3 tons dry matter per acre with a Relative Forage Quality index of 180; removing 25 to 45 pounds of P2O5 per acre reducing soil test values by 2 to 4 pounds. They also are excellent at scavenging excess soluble nitrogen and phosphorus from the soil before it enters any water body. The cover crop is doing this outside the growing season for corn and soybeans. The oat forage-cover crop will need to be harvested in the late fall and will yield about the same as the winter annuals. The highest yields will come from the earliest planted cover crops. Corn and soybean herbicide programs need to be reviewed to make sure they do not interfere with the establishment of the cover crop.
Harvesting these forages in the boot stage will balance both yield and feed quality and will eliminate the need to kill the cover crop before planting the next crop. Boot stage is just before the seed head emergence when the seed head can be felt near the top of the plant at the whorl of the flag leaf. The boot stage is a very short period lasting only a few days. Forage quality drops quickly after the boot stage loosing 4 to 5 Relative Forage Quality points per day. It is better to harvest too early than too late. Baleage is the recommended harvesting system. Custom harvesting is an excellent option for farms with heavy spring workloads. If the forage is not needed on the farm it can become an excellent cash crop.