Nutrient production model

Nutrients in the soil, bedding and feed residue are summed to determine total nutrient production (equation 3). Upon manure excretion by the animal, nitrogen will volatilize in the form of ammonia, and phosphorus may be lost through runoff. A nutrient retention factor is necessary to account for losses to represent as-removed manure.

Nutrient production is calculated through equation 3:

NP = R x ( N_e + N_b ) + N_s

Where:

• NP is nutrient production (g head-1 d-1);
• R is retention coefficient (decimal);
• Ne is nutrients excreted (g head-1 d-1);
• Nb is nutrients in bedding (g head-1 d-1); and
• Ns is nutrients in soil (g head-1 d-1).

Nutrients excreted

Nutrients excreted are provided by the ASABE model (2005). For the nine example diets, nitrogen excretion ranges between 144 and 205 g head-1 d-1, and phosphorus excretion ranges between 2 and 32 kg head-1 d-1.

Nutrients in bedding

Nitrogen and phosphorus in bedding will vary according to type of bedding and moisture content. Nitrogen content of wheat straw is 6.2 kg t-1 while barley straw has 6.7 kg t-1 (dry basis). For phosphorus, both barley straw and wheat straw have 0.8 kg tonne-1 (CowBytes). Moisture content can range between 10 and 14 per cent or greater.

Nutrients in soil

For the model it is assumed that in soil nitrogen is 7.5 kg t-1 and phosphorus is 1.5 kg t-1.

Retention coefficient

The retention coefficient is applicable only to bedding and feed residue, because the properties of feed and bedding inputs are known at the time they are added to the pen. The retention coefficient accounts for losses between addition of inputs and the time of manure removal. For the model it is assumed that nitrogen retention varies between 40 and 50 per cent, and that phosphorus retention varies between 70 and 90 per cent.

Observed nutrient production versus the nutrient production model

Nutrient production was calculated for each sample diet with the following assumptions: nitrogen retention ranges between 40 and 50 per cent; phosphorus retention ranges between 70 and 90 per cent; bedding addition ranges between 0.3 and 0.7 per cent animal weight per day; bedding has a moisture content of 12 per cent, bedding has nitrogen content of 6.2 kg t-1 and phosphorus content of 0.8 kg t-1; soil removal ranges between 0 and 4 kg head-1 d-1; nitrogen content of soil is 7.5 kg t-1 (wet basis); and, phosphorus content of soil is 1.5 kg t-1 (wet basis).

Equation 3 predicts that as-removed nitrogen production ranges between about 60 and 143 g head-1 d-1, and phosphorus production ranges between 20 and 37 g head-1 d-1. This compares favourably to the data observed in Saskatchewan where nitrogen production ranged from 51 to 155 g head-1 d-1 and phosphorus production ranged from 10 to 37 g head-1 d-1 (See Saskatchewan references - table 3).

Nitrogen concentration

Nutrient concentration is equivalent to nutrient production divided by manure production (equation 1), and it represents the proportion of nutrients present in manure.

Each diet in Table 5 (Manure production model) was given to the existing ASABE model for feedlot manure, and this resulted in nine different values for daily dry matter and nutrient production. The values given to the model for moisture content were 30, 50 and 70 per cent. The values given to the model for bedding are 0.3, 0.5 and 0.7 per cent of bodyweight. The values given to the model for soil removal are 0, 2 and 4 kg head-1 d-1. The values given to the model for nitrogen retention are 40 and 50 per cent. This resulted in 243 different values for manure production and 486 different values for nutrient concentration (9 x 3 x 3 x 3 x 2).

Nitrogen concentration plotted against manure production (figure 1) finds values predicted by the model are very similar to those observed in Saskatchewan. As manure production increases nitrogen concentration decreases, because excessive manure production is attributable to soil, bedding and water. These constituents contain minimal nutrients.

Phosphorus concentration

Phosphorus has a similar relationship to manure production as nitrogen (figure 2). Maximum phosphorus concentration predicted by the model is about 5 kg t-1 (10 lbs ton-1), whereas the full dataset collected in Alberta and Saskatchewan saw a few instances of phosphorus between 5 and 10 kg t-1 (10 to 20 lbs ton-1). Phosphorus losses predicted by ASABE (2005) and Kissinger et al. (2007) range between nine and 15 per cent, while losses observed in Saskatchewan range between 10 and 30 per cent. It is possible that the difference in phosphorus concentration between the model and the observed values results from the example diets containing different phosphorus content than that actually fed.

Bedding

The impact of bedding on manure properties is often a source of confusion. Bedding is often blamed if manure samples cannot be explained, or if book-values differ between regions. For a calf fed 386 to 590 kg (850 to 1,300 lbs) in 150 days, assuming nitrogen loss of 50 per cent, phosphorus loss of 10 per cent, moisture content of 53 per cent, and soil removal of 2 kg head-1 d-1, table 7 shows the change to manure properties as bedding use increases.

Table 7. Impact of bedding on manure properties.

The addition of bedding significantly increases manure production while total nutrient production remains nearly the same. When manure production is high nutrient concentration is low.

Error in predicting as-removed manure properties results mostly from nitrogen and phosphorus losses. The quantity of nitrogen and phosphorus loss is unknown and varies significantly. This model estimates that nitrogen loss varies between 50 and 60 per cent and phosphorus loss varies between 10 and 30 per cent. These estimates are in accordance with observations in Saskatchewan and similar to values provided by references.

Manure is not homogenous. Any particular sample may not be representative of the entire pen. Differences occur in bedding concentrations across the pen profile. Moisture conditions vary throughout the vertical profile of the manure pack and the horizontal profile of pen area. Any truckload of manure can be different from other truckloads.

The model is applicable to bulk properties of manure. For the entire pen or feedlot, properties of manure will approach average values.

The model seems to predict feedlot manure characteristics with sufficient accuracy for practical purposes. The assumptions are reasonable, in accordance with references, and are supported by data observed in Saskatchewan. Provided a feedlot operator keeps accurate records, and moisture content and bedding usage is known, manure properties for any given pen can be predicted.