Manure production model

Mass of manure production can be calculated by estimating the amount of dry matter in manure, correcting for moisture content and accounting for soil removal (equation 2). This model assumes that feed spillage is negligible.

Manure production is calculated through equation 2:

MP = Sr + (DMe + DMb)
                  (1 - MCm)

Where:

• MP is manure production (kg hd-1 d-1);
• DMe is dry matter, excreted (kg head-1 d-1);
• DMb is dry matter, bedding (kg head-1 d-1);
• MCm is moisture content, as removed manure (decimal); and
• Sr is soil removed, wet basis (kg head-1 d-1).

Dry matter excretion

Dry matter in manure resulting from feed can be estimated with the use a mathematical model, such as the one provided by ASABE. Dry matter excretion for several typical feeder cattle diets might range between 2.1 and 3.7 kg head-1 d-1 (table 5).

Table 5. Dry matter excretion as predicted by ASAE D384.2, Section 4.0.

[a] DMI means dry matter intake
[b] CP means crude protein
[c] TDN means total digestible nutrients

Example diets in table 5 are based upon rations of barley, alfalfa and mineral supplements providing weight gain of 1.4, 1.6 and 1.8 kg d-1 (3, 3.5 and 4 lbs d-1).

Estimating contribution of bedding

The amount of bedding use will vary according to climate. This model assumes that bedding usage ranges between 0.3 and 0.7 per cent of the animal's bodyweight. From "Beef Cattle Feeding Systems" (2004), bedding usage is about 2.2 kg head-1 day-1. The moisture content of bedding will vary according to climate and type of bedding, and moisture content between 10 and 14 per cent is a reasonable estimate for wheat straw.

Estimating contribution of soil

Even if the pen floor remains intact during manure removal, soil is mixed into the manure pack by hoof action and will be removed. The amount of soil removed with manure may vary significantly. If 2.5 cm of soil is removed during yearly cleaning, this would be approximately 2 kg head-1 d-1 at stock density of 18.6 m2 head-1 (200 ft2 head-1) and soil density of 1,620 kg m-3.

Kissinger et al. (2006) found that manure removal was 9.6 kg head-1 d-1 during spring cleaning versus 4.8 kg head-1 d-1 in the fall, and the difference was attributed to soil. Gilbertson et al. (1975) found that soil removal ranged between 2.0 and 4.7 kg head-1 d-1. The contribution of soil to manure can be significant.

Actual soil removed from a feedlot is heavily dependent on the particular operator and specific feedlot conditions. With experience, a feedlot may estimate actual soil removal based upon the amount of fill required to replace pen floors. Lacking better information, 0 to 4 kg head-1 day-1 is a reasonable estimate of soil removal.

Observed manure production versus the manure production model

The model predicts that in Saskatchewan, manure production could range from 4.7 to 25.1 kg head-1 d-1 (table 6). The range of values observed in Saskatchewan is 5.5 to 28.2 kg head-1 d-1 (See Saskatchewan references - table 3). The model seems to predict manure production somewhat accurately.

Table 6.  Manure production predicted by model.