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Are you getting the most from your manure?

June 22, 2011
Not all manures are created equal. Knowing their differences and when to use them can help you get more value out of your applications.
Manure is a valuable resource! And it is hard to argue with this statement, because there is a huge variation of the value of manure to different operations. However, applying some knowledge about the nature of the material can help you to capture a bit more of that value for yourself.
The first thing to realize is that not all manures are created equal. The nitrogen (N) in manure is divided between the ammonium form, same as fertilizer nitrogen, and the organic form, which must break down to release the mineral nitrogen before crops can use it. In general, liquid manure has a higher proportion of ammonium N than solid manure. Within these classes, manure from cattle and sheep has the highest proportion of organic nitrogen, while swine and poultry manure has more ammonium nitrogen. The amount of bedding materials used and the treatment and storage of the manure can influence these proportions. For example, digested manure will have a much higher ammonium N proportion than the raw manure entering the digester.
Optimum application timing:
While applying manure will not make much difference to the availability of phosphorus or potassium from the manure, it will make a big difference to nitrogen. Liquid swine manure, which has a lot of ammonium N, will have greatest N availability if it is applied close to the time the crops need the nitrogen. This may be immediately pre-planted in the spring, or even side-dressed into standing crop.
Optimum placement:
Getting manure quickly covered with soil will help retain ammonium nitrogen, irrespective of the manure type, but the relative amount of ammonium and organic nitrogen does have a big impact. If the manure (or compost) has very little ammonium nitrogen, then losing a large portion of ammonium by leaving the manure on the surface will have little impact on the amount of N that is available to the crop. Injection, or immediate incorporation, is much more important for liquid manure with high ammonium N content.
Retaining the ammonia from a spring application translates directly into reduced fertilizer N requirement, while retaining ammonium in the fall may just be trading one form of nitrogen loss (volatilization) for another (leaching or denitrification).

 Optimum field selection:
The field which benefits the most from manure application is one growing a crop that can use all of the nutrients applied in manure. Traditionally, this has meant a crop like corn on and field that was low on phosphorus (P) and potassium (K). Choices have to be made between fields with a broader range of crops and nutrients requirements. Take the example of a farm with liquid dairy manure, applied in the spring at a rate of 5,000 gallons per acre. A crop that could use all of the nutrients in this application would get a benefit of $193 per acre, but this farm has one field with very high P and K soil tests, which is growing corn, and another field at the back of the farm growing alfalfa, but with very low soil tests.
The value of the manure on the corn would only be $58 per acre, since there would be no direct value of the P and K, while there would be a value of $135 per acre on the alfalfa even though it does not need any of the nitrogen. On this farm, the manure should be applied to the alfalfa first and only the left-over manure to the corn ground.
Another difficult factor to quantify is the value of the organic matter in the manure. Fields low in organic matter will generally show greater response to added manure, both in improved soil quality and in crop yields, but these increases are not as consistent as nutrient responses.
Optimum rate:
The biggest challenge with manure, aside from the handled volume, is that you cannot change the nutrient concentrations to match crop requirements. There is also variability of nutrient content and availability within the manure and variation in application rates.
Growers who depend completely on manure to meet their crop requirements for nitrogen are often over-applying phosphorus and potassium, and may still fall short of the N required in some parts of the field.
One technique to overcome this is to use manure for only a portion of the N rates (up to 75 per cent), and balance the rest with mineral fertilizer. This combination will often generate higher yields than either manure or fertilizer on its own.
Source: Keith Reid, Ontario Ministry of agriculture, Food and Rural Affairs.



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