Manure can take nutrients the full circle
GroundCover™ Issue: 56
Feedlot manure is a source of almost all the nutrients required by plants. Dr Peter Wylie examines the potential for expanding its use.
[Photo: Manure from a feedlot can be partially composted to improve spreadability (left) and then spread on the paddock (right)]
Manure from the expanding feedlot industry is a potentially large and valuable source of nitrogen and phosphorus for farm soils. Feedlot manure is a source of almost all the nutrients required by plants. It can supply a balanced mix of nutrients in a slow-release organic form, at a lower cost than alternative fertilisers.
There is an additional benefit of adding organic matter to the soil. Beef cattle numbers in feedlots are at record levels, with a major concentration in southern Queensland where about 400,000 are on feed. With around 1.2 tonnes of manure produced from each feedlot operation, this means about 500,000 tonnes of manure each year.
There is about five kilograms of phosphorus in each tonne of feedlot manure, therefore the amount of feedlot manure currently produced is enough to replace normal applications of 8kg of phosphorus fertiliser per hectare on about 300,000 hectares of farming land each year.
When priced at the value of commonly used fertilisers, the nitrogen and phosphorus content of feedlot manure is currently worth around $23 a tonne for fresh manure that has about 5kg of phosphorus per tonne, and $27/t for aged or composted manure that has 7kg of phosphorus per tonne. The nitrogen content of manure is generally around 16kg/t.
The cost of manure spread on to farms, with around $10 of freight and $5/t spreading costs, is close to $20/t. This means the cost is comparable to other fertilisers when farmers apply nitrogen and phosphorus fertilisers. A disadvantage is paying a lot up front for several years" worth of nutrients.
But when the value of potassium is considered, the manure is valued at $37/t for fresh manure and $43/t for aged manure. Sulfur and zinc can add an extra $1.50/t in nutrient value where these nutrients are of value.
Taking all these nutrients into account, manure can replace nutrients in the soil for around half the cost of mineral fertilisers, such as urea, MAP and muriate of potash. This means the use of manure is particularly valuable in situations where potassium is needed.
Until recently, potassium has not been an issue. It is a nutrient required by plants in similar quantities to nitrogen. However, most of this remains in the leaf and straw and only three to four kilograms of potassium is removed per tonne of grain. This adds up however, with more than 200 tonnes of grain being harvested from soils on the Darling Downs cultivated for 50 to 70 years. This means a removal of 600 to 800kg of potassium without any replacement.
Cotton is one of the most demanding crops with respect to potassium. Deficiency shows up as red leaves and premature senescence. Problems are starting to show, and irrigated areas, which have high removal rates, now need potassium fertiliser to ensure top yields.
Another useful way to approach the use of manure is to apply it as a phosphorus fertiliser with 7.5t/ha of aged feedlot manure or 2.5t/ha of poultry manure providing 50kg of phosphorus.
Fifty kilograms is enough phosphorus to replace or match the removal of 15 to 18 tonnes of grain or 20 bales of cotton. If sorghum yields 5t/ha, it will remove around 15kg of phosphorus. Seven and a half tonnes of aged manure with 50kg of phosphorus will therefore last three years to match the phosphorus removal rate.
It is common for phosphorus fertiliser not to be used at replacement levels, and a common application rate of 8kg of phosphorus/ha would last up to six years.
Plenty of phosphorus and potassium should be available in the first year after application if the manure is incorporated into the soil by cultivation. The problem is predicting the release of nitrogen to the soil to decide how much nitrogen fertiliser is needed to supplement nitrogen from the manure.
The nitrogen in manure is an organic form, which has to be decomposed for it to be released for a crop. Under dry weather situations, nitrogen should be applied at close to normal rates in the first year after application of manure. Under better moisture conditions, between 30 and 40 per cent of the nitrogen in manure may become available in the first year, provided the manure is incorporated several months before the crop is planted.
An application of 7.5t/ha of manure might have a total of 120kg of nitrogen, with 30 to 50kg of nitrogen becoming available to the next crop. If the crop is sorghum with a yield target of 5t/ha and 90kg of nitrogen is normally applied as fertiliser, then a top up with 50kg of nitrogen is needed.
Depending upon conditions in the first year, less than a third of the nitrogen is likely to become available in the second year, and only 15 per cent in the third year, requiring an appropriate top-up of nitrogen to grow good crops in the second and subsequent years.
Application of manure to crop fields is perhaps the biggest stumbling block for good results in replacing mineral fertilisers. Fresh manure is often lumpy because the yard scrapers tear up manure compacted by many hooves. Spreaders will often feed lumpy manure out in bursts and do not spread it across the swath evenly.
This is why manure should be aged and screened for good distribution, particularly if low application rates of five to 6t/ha are being used. For unscreened manure, application is hit and miss at rates below 10t/ha.
Feedlots using screening plants will often stockpile manure for up to a year to allow it to decompose and become friable enough to screen. But ageing for 12 months results in the loss of almost half of the nitrogen and organic matter, almost as much nutrient as composting.
Partial composting is a compromise which minimises nutrient losses and the cost of manure handling. If manure is taken from the yards and turned two or three times in windrows over four to six weeks, it will use up any excess moisture, while mellowing the manure so it can be screened or spread.
The nutrient content of manures is variable, with some of the differences due to weather, age and timing of manure collection.
Table 1: Content and value of nutrients at current fertiliser prices
1: Powell, 1994. Economic management of feedlot manure, final report, (average of 93 samples, 17 stockpiled for more than 12 month)
2: Recently composted manure - corutesy of Environoganics
3: Average of several analyses from Pittsworth-Millmerran Poultry Farms
One way to use manure is to apply it as a phosphorus fertiliser with 10t/ha of aged feedlot manure or 3.5t/ha of poultry manure providing 70kg of phosphorus.
Seventy kilograms is enough phosphorus to replace or match the removal of 20 to 25t of grain from a paddock. Figures of removal can be used to monitor the time interval for manure applications.
For example, with sorghum yielding 5t/ha, aged manure applied at 10t/ha would last four years to match the phosphorus removal rate (70kg of phosphorus applied and four years x 16kg of phosphorus removed). With irrigated cotton at nine bales per hectare using 23kg of phosphorus/year, 10t of manure would provide enough phosphorus for three years.
For more information: Dr Peter Wylie, Principal Consultant, Horizon Rural Management, 07 4662 4899, email@example.com
Region North, South, West