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Properties of liquid dairy manure in Saskatchewan

Book-values are helpful to livestock producers, nutrient management specialists, and regulatory agencies. However, no single book-value can represent manure at all sites across the province at all times. Manure properties are subject to tremendous variation and therefore book-values represent average conditions. Especially for new operations without site specific data, book-values are very important for constructing manure storage with adequate volume and securing adequate land for the sustainable use of nutrients available in manure.

On this page, units of nutrient concentration are given as grams per 10 litres. This is equivalent to pounds per 1,000 imperial gallons.

Existing data collected in Saskatchewan

In 2014, both volume of manure production and nutrient concentration was collected from six dairies in Saskatchewan. The size of this dataset is small and therefore caution should be exercised in drawing conclusions. For this reason, nutrient data from nine dairies in Manitoba is included in this analysis.

In Saskatchewan, total nitrogen concentration ranges from 26 to 42 g/10 litres and the average is 33 g/10 litres. Phosphorus concentration ranges from 3.6 to 8.1 g/10 litres and the average is 5.6 g/10 litres. Potassium concentration ranges from 18.3 to 28.7 g/10 litres and the average is 22.1 g/10 litres. Moisture content ranges from 88.6 to 96.2 per cent and the average is 91.9 per cent.

There is a strong correlation between percentage of nitrogen (dry weight) and the percentage of solids (Figure 1). This observation is significant: if solid content is known, then concentration of nitrogen in liquid manure can be predicted (Equation 1).

Nitrogen Concentration vs Solid Content
Figure 1 – Nitrogen Concentration vs Solid Content

Equation 1: Nww = Ndw x S x (1 + 0.0041 x S)

Equation 2: Ndw = 24.769 * S-0.857

where:

  • Nww is nitrogen concentration g/10 litres;
  • Ndw is nitrogen concentration per cent dry weight; and
  • S is solid concentration per cent.

For the purpose of establishing book-values, a reasonable estimation of solid content can be calculated if both volume and dry matter are known. Dry matter excretion for a typical animal can be calculated from equations provided by the American Society of Agricultural Engineers (ASAE). Volume is provided by manure production values measured in Saskatchewan.

Volume of manure production

Volume of manure production was measured at six dairies in Saskatchewan. At three of the dairies, only milk cows contributed to the manure storage. At the other three dairies, associated livestock also contributed. Due to the small dataset, no difference in manure production between the two production systems was noticeable. Milk house waste was included in all cases.

Volume of manure production ranges from 0.145 to 0.191 L/kg animal/d. The average is 0.166 and the median is 0.161 L/kg animal/d. Therefore, a reasonable range of manure production for a 669 kg milk cow is between 97 and 128 L/day.

The range of manure production is large. Volume of manure production is influenced by many factors, including type of milking system, separation of solids, amount of wash water and distance from bulk tank to milk house. For design purposes, specific management practice should be considered.

On this page, volume of manure production is given in terms of the animal’s unit weight. Cow weight is a very important consideration that is sometimes overlooked. Depending on the dairy farm, an average milk cow could weigh between 1,375 and 1,625 lb (624 to 737 kg). This has a significant influence on volume of manure production.

Book-values are often given in terms of volume per animal. This is appropriate for hogs, beef and poultry, where the animals tend to be finished to the same weight. For dairy, the book-value for manure volume should reflect the weight of the animal.

Manure properties from models

Dry matter excretion can be calculated from dry matter intake with equation 5.3.4(8) ASAE (Standard D384.2 2005). Dry matter intake can be calculated from the National Research Council’s “Nutrient Requirements of Dairy Cattle” equation 1-2. This requires that weight of cattle and milk production are known.

It is acknowledged that cattle weight and daily milk production will vary. Throughout the lactation period, body weight first decreases and then increases, while milk production tends to decrease. For the purpose of predicting dry matter excretion, an average body weight and average milk production is selected.

Each producer involved with Saskatchewan’s study was asked to describe the weight of their animals. While the dataset is small, a reasonable estimate for the weight of an average milk cow is 1,475 lbs (669 kg). A reasonable assumption for average milk production at this time was 32 kg/cow/day.

Plaizer et al (2004) collected data from 40 dairy farms in Manitoba and found that median milk production was 29.3 kg/cow/day. If milk production increased about 10 per cent in 12 years, then 32 kg milk/cow/day is a reasonable assumption.

From the model given by ASAE for a 669 kg milk-cow and 32 kg daily milk production, it is estimated that dry matter intake is 22.0 kg/cow/day and dry matter excretion is 8.73 kg/cow/d.

Moisture content

Volume of manure production is dependent upon moisture content. In other words, greater water usage results in a larger volume of manure. If the amount of dry matter and moisture content are known, then volume of manure can be calculated from Equation 3:

DM

V = -----------------------------------------------

1.41 – 1.82 MC + 0.41 MC2

Where:

  • V is volume of manure in litres;
  • DM is mass of dry matter in kilograms; and
  • MC is moisture content as a decimal.

Equation 3 is derived from the following principles:

(a)    solid content is the opposite of moisture content;

(b)    solid content is the portion of solid matter in a given quantity of manure by weight; and

(c)     volume of manure is determined from mass by the specific gravity of manure. Specific gravity of manure is given by equation 4 (Tunney and Bertrand, 1989):

𝑆G = 1 + 0.41 · SC

Where:

  • SG is specific gravity of manure in kg/L; and
  • SC is solid content as a decimal.

Figure 2 shows the relationship between volume of manure production and moisture content predicted by Equation 3 for a 1,475 lb (669 kg) milk cow. This assumes dry matter excretion of 8.73 kg / day and includes the addition of 0.5 kg dry matter from bedding.

Volume of manure vs moisture content
Figure 2 – Volume of manure vs moisture content

From observations in Saskatchewan, volume of manure production for a 1,475 lb (669 kg) lactating cow ranges between about 97 and 128 L/animal/day. Typical moisture content therefore should range between about 91 and 93 per cent (from Figure 2 and Equation 3).

Nitrogen concentration

From Equation 1, nitrogen concentration will range between about 33.6 and 35.2 g/10 L for moisture content between 91 and 93 per cent (solid content between seven and nine per cent). This is slightly higher than the average value of 32 or 33 g/10 L from the dataset.

An alternate means of calculating nitrogen concentration is from mathematical models. First nitrogen excretion is predicted from known production characteristics. Then concentration of nitrogen in as-removed manure is adjusted for nitrogen loss in the form of ammonia.

Nitrogen excretion can be predicted from ASAE standard D384.2 equation 5.3.7(14) if the following are known: milk production, days in milk, dry matter intake, concentration of crude protein, and bodyweight. Plaizer et al (2004) collected data from 40 dairy farms in Manitoba and found that average days in milk were 171 and concentration of crude protein was 18.3 per cent.

Given the above assumptions, the ASAE model predicts that nitrogen excretion is 456 g/lactating cow/day. Figure 3 shows the difference in nitrogen between as-excreted and as-removed manure (as-removed manure is described by Equation 1).

Nitrogen Concentration vs Manure Volume
Figure 3 - Nitrogen Concentration vs Manure Volume

Nitrogen loss

Nitrogen is lost from manure in the form of ammonia gas. Several factors influence ammonia release, including temperature, ventilation rate, air speed, moisture content, surface area, storage time, nitrogen content, and pH.

Several studies were examined in order to determine a reasonable range for ammonia loss from dairy manure. Kulling et al (2003) found that total nitrogen loss was 19 per cent for liquid manure and 30 per cent for slurry. Sparks et al found that total nitrogen loss ranged from about eight to 18 per cent for scraped manure in 24 hours. Aguerre et al estimated that nitrogen loss ranges between 18.2 and 20.7 per cent in 136 days. Krober et al (2000) found that nitrogen loss increased from 10.7 to 23.7 per cent as crude protein increased from 12.4 to 17.5 per cent.

Summary of book-values

A summary of book values for liquid dairy manure are presented in Table 1. For Potassium, ASAE does not offer equations for predicting potassium excretion. The value for potassium is calculated from 29.2 g potassium/kg dry matter from the dataset collected in Saskatchewan.

Table 1 – Summary of book-values for liquid dairy manure.

Type

Weight (kg)

Volume (L / hd / d)

Moisture Content (%)

N

(g / 10 L)

P

(g / 10 L)

K

(g / 10 L)

Lactating cow

669

115

92

34

5.6

22

Dry cow

669

57

88

33

6.7

34

Heifer

519

45

88

33

6.6

34

Calf

206

18

88

32

6.5

34

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