Table of Contents
Forage Fertilization
Moderate to high forage yields are necessary in forage systems to cover overhead costs. To maintain profitability forage managers need to ensure that fertilization practices allow adequate yield without wasting money on unneeded fertilizer.
If too little fertilizer is used, forage yield will be reduced. When making hay, machinery and labor costs per ton of hay will be increased if adequate fertilizer is not used, since less hay will be made for the same effort.
If too much fertilizer is used, cash cost per ton of forage increases since the crop can only use a limited amount of nutrients. In some cases there may also be negative effects such as nitrates moving into springs or wells putting family members and livestock at an increased health risk.
So how much fertilizer do I need? The potential forage yield from a field is determined by the available water holding capacity of the soil in the field.
Soil Water Holding Capacity Determines Yield Potential.
The actual forage yield from a field will be determined by the crop grown, weather, soil fertility, fertilizers applied, and harvest management.
This tutorial will cover the effects of soil fertility and fertilization on forage production. Charts will be used to quantify the expected forage growth due to soil fertility or fertilizer.
Why use charts? Well, are you a gambler? If you are willing to take risks (being a gambler) farming is the place to be. Yield response charts show the odds you face when making fertilization decisions.
These charts use relative yield and not actual yield in tons per acre. Relative yield is used since soil type (water holding capacity of the soil) determines potential yield. Using relative yield allows combining research results from trials conducted on different soils, having different potential yields.
To convert a relative yield of 0.8 to a field yield for a soil having a potential yield of 4 tons/acre, multiply the relative yield by the potential yield. Example: 0.8 x 4 = 3.2 tons/acre
Many of these charts have a horizontal and a vertical line on them. The horizontal line represents a relative yield of 0.9 (90%) of potential yield. In field research, yield differences often have to be greater than 10% for them to be significantly different. The vertical line marks the level of soil fertility or fertilizer application that ensure yields will be kept above the 0.9 relative yield level.
Back to Forage Fertilization.
There are two aspect of forage fertilization:
Soil Fertility Effects on Forage Production. (All soil test values are based on West Virginia University Soil Testing Laboratory Methods.)
Phosphorus (P) - The following chart shows the effect of differing levels of soil test P on the relative yield of forage crops. When soil test P is less than 50, yields can be expected to be below potential. As soil test levels drop below 40, yields drop off rapidly.
Forage Yield Response to Soil Test P.
When soil test P is above 50, no P fertilizer is needed to maintain production; for short periods of time. However, if no P is applied the crop removing P will reduce soil P, resulting in reduced yields as the soil test decreases below the critical value. Forage crops remove 12 pounds of P fertilizer (P2O5) per ton of dry forage removed from the field.
Potassium (K) - The following chart shows the effect of differing levels of soil test K on the relative yield of forage crops. When soil test K is less than 120 yields can be expected to be below potential. As soil test levels drop below 120 yields may drop off rapidly.
Forage Yield Response to Soil Test K.
When soil test K is above 120, no K fertilizer is needed to maintain production; for short periods of time. However, if no K is applied the crop removing K will reduce soil K, resulting in reduced crop yields as the soil test decreases below the critical value. Forage crops remove 45 pounds of K fertilizer (K2O) per ton of dry forage removed from the field.
Fertilization Effects on Forage Production
Supplemental P and K fertilizers are needed to replace fertility removed in harvested crops and to increase plant nutrients in soils low in fertility.
When the soil test is maintained in the medium to high range, supplemental nutrients only need to be applied at the removal rate in order to maintain yields near the maximum. The following chart shows this for P.
Fertilizer P Affect on Forage Yield When Soils Test Medium to High in P.
However, when soil test P drops into the low range higher rates of supplemental P are needed since part of the P is held by the soil and is less available to plants. The following chart shows how on soils testing low in P, supplemental P at up to 4 times the removal rate may be needed to obtain maximum yield.
Fertilizer P Affect on Forage Yield When Soils Test Low in P.
Crops grown on soils testing medium to high in K will produce their maximum yield when K is supplemented at the crop removal rate.
Fertilizer K Affect on Forage Yield When Soils Test Medium to High in K.
Crops grown on soils testing low in K only need supplemental K at the crop removal rate, since soils in our region do not hold K in forms not readily available to plants. This is different than what we saw for P.
Fertilizer K Affect on Forage Yield When Soils Test Low in K.
Remember that Fertilizer Needs Are Determined by Forage Yield! A ton of hay removes:
Both pasture and hay crops need proper fertilizer supplementation. The difference is that in pastures livestock return most of the fertility through their dung and urine. However, this is not always distributed uniformly over the pasture so strategic soil sampling and fertilization may be needed.
Example Hay Fertilizer Needs:
Where historic field yields are not know, use the published soil yield from the county soil survey as a guide to how much yield can be expected from the soil when under good management. Compare historic yields to the published soil yield to see how well you are doing compared to what you might be able to do. Under very good management, some producers achieve 25% above the soil survey yield values.
Nitrogen Fertilizer
Forage crops need a source of nitrogen (N) for good growth as well as P and K. N is used by both plants and animals in proteins and for enzyme systems needed for growth and reproduction.
Nitrogen can be provided by:
In grass-legume stands 30-60% legume is needed for high production.
Legume Content Affects Forage Yield
The following images show stands with different legume contents.
PPT Slide
PPT Slide
PPT Slide
Properly managed grass-legume hayfields and pastures can produce as much forage as the grass fertilized with 150-200 lb N /acre/year, which would cost $45-$75 /acre/year.
For good legume production adequate lime needs to be applied to maintain soil pH in the range suitable for the legume being grown.
Soil pH Affects Legume Yield.
In grass stands where there is little legume N from commercial fertilizers, livestock manure, poultry litter, or biosolids can maintain increased forage yields. The following chart shows the effect of N rate on relative yield of perennial grasses in Northern WV.
Forage Yield Response to Fertilizer N.
When applying N to forage crops, apply between 50-60 lb of N/acre/cut, depending on your needs and local economics, up to a maximum of 180-220 lb/acre/year. When N is used for stockpiling tall fescue, up to 100 lb/acre can be applied in August as long as the total annual N rate does not exceed 180-220 lb/acre/year.
The upper limit of N fertilization on forage crops should be held between 180 and 220 lb. available N/acre/year from all sources. This includes crop residues and residual soil organic matter from poultry litter, manure, or bio-solid applications in previous years. If more than 180-220 lb. N/acre/year is available there is the risk that nitrates will move below the rooting zone, contaminating the water table.
The cost effectiveness of N fertilizer on pure grass stand can be estimated from the following chart. The response of N applied to mixed grass-legume stands is about half or less of that obtained from straight grass stands depending on the legume species and content and on the organic matter content of the soil.
Forage Yield Response per Pound N
The previous chart shows that when applying 100 lb N/acre/year there will be an additional 20-40 lb of forage dry matter produced for each pound of actual N applied, depending on weather and other field conditions. If N costs $0.30/pound and the response is 30:1 then the additional forage will cost $0.01/pound or $20/ton.
Nitrogen Fertilization for Hay:
Remember that the fertilizer ratio needed for grass-legume hay where no N is needed is: 0-12-45 100 lb/ton hay removed. This same ratio should be used for N-fertilized grass hay with the addition of 50-60 lb N/acre/cut.
Summary
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