By Dr Peter Wylie
Feedlot manure has some pros and cons in a dry year, but its major value comes to the fore in a wet year.
In a dry year, very little of the organic nitrogen in the manure will be released, but that is usually not a problem, because demand for nitrogen by crops is low. The big plus for manure is in a wet season, when the extra nitrogen mineralised could boost sorghum yields by as much as 2t/ha.
Phosphorus (P) should be available in a dry year, unless it has been recently applied and is mostly on the surface. Applying manure to ground after harvest when it is cracked, will help this problem. Only 10-20% of the manure will go down the cracks, but if 70kg of P is being applied in 10t/ha of feedlot manure, this is still quite a bit of P. Some farmers use some starter P in the early years to make sure there is enough P, but particularly after the second application of manure, Starter P should not be needed.
This can lead to manure being a help to cash flow, with decreased purchases and/or spreading in dry years, when cash is short, while crop demand for P is still being met.
A frequent observation of farmers is that they see no difference from applied manure. This can occur when little or no nitrogen is released in a dry season. It can also occur because the soil which did not receive manure has enough residual P from 30 or more years of starter fertiliser, so no differences will show in the first two or three years that starter P is discontinued. Where manure is applied to deficient soils, the response can be impressive.
Over time, feedlot manure can save money, with a value around $50/t, while poultry (layer) manure is close to $70/t, when nitrogen and phosphorous in the manure are valued at the unit costs of Urea and MAP. Extra value comes from potassium (K), sulfur, zinc and organic C.
Table 1: Value of nutrients at fertiliser prices.
|Aged Beef Manure||Poultry Manure (Layers)|
|Water content (variable)||30%||33%|
|Phosphorus (P) (kg/t)||7||13|
|Potassium (K) (kg/t)||20||11|
|Value of N and P*||$50||$70|
|Value of N, P & K*||$80||$100|
|Cost at source: ($/t)||$12||$20|
|Cost with Freight $13, and spread $5||
|Extra value compared to bag fertiliser ($/t)||
Poultry manure from egg layers is more concentrated than feedlot manure, but has less organic matter and potassium. Broiler shed manure is variable, depending upon how much sawdust is mixed with the bird droppings. Other recycled organics from composts of sewage sludge and piggeries are worthy of consideration – based on nutrient content.
One way to use manure is as a P fertiliser and to add extra nitrogen as needed.
10t/ha of feedlot manure has 70 kg of P, enough for 20t/ha of grain removal (at 85% recovery and 3 kg P/t of grain).
By example, sorghum yielding 5t/ha would have a removal rate of 15 kg of P per year and 10t/ha of manure would last four years. Irrigated cotton yielding 10 bales/ha will use 25kg P/year, and 10t/ha of manure is only enough for 2 crops. Cotton has a high K requirement, with some 60kg/ha removed in 10 bales/ha.
Nitrogen has to be managed according to the season. 10t/ha of manure may apply 160kg N/ha but only 30% to 40% of this N is available in the first year, and then only if application is several months before the first crop is planted and there is good rainfall before and during the crop.
More N will be released in summer than in winter, while the N release increases with the rainfall which is good for sorghum. From 10t/ha of aged feedlot manure, 50-70kg N might be released in year 1, with 25-50kg N available for the second crop after spreading.
If manure is to be used as a fertiliser, it needs to be evenly distributed and should be aged and screened before use. A mixture of dry manure and screened lumps will spread to 9 or 10 metres, with 20% overlap. Spreaders should be calibrated, and spinners adjusted for this.
Incorporation is not required for aged manure as most of the nitrogen is in a nitrate or organic form, and only a small amount of ammonium N will be lost.
Spreading on dry, cracked ground is a good opportunity to have nutrients moved to some depth – by rain before the cracks close. Manure is also likely to encourage earthworms which also move nutrients downwards.