For the purpose of book values, a single value must be selected. Manure production from a 669 kg cow of 115 L/lactating cow/day (0.164 L/kg cow/day) is in the middle of the observed range in Saskatchewan. This volume of manure has a moisture content of approximately 92 per cent (Properties of liquid dairy manure in Saskatchewan - Equation 3).
Nitrogen loss is about 15 per cent (from Properties of liquid dairy manure in Saskatchewan - Figure 3) and nitrogen concentration is about 34 g/10 L manure.
Each of these values is near the middle of the range and each value seems reasonable.
Phosphorus
Kaizer et al (2004) found that in Manitoba, phosphorus concentration fed to milk cows is 0.48 per cent of dry matter. From this, the ASAE model predicts that phosphorus excretion is 80.4 g/lactating cow/d. This results in phosphorus concentration of 7 g/10 L manure at manure production of 115 L/lactating cow/d.
This concentration falls toward the high end of the range observed in the dataset. Two reasonable explanations for this difference are:
- The ASAE model may over-estimate phosphorus excretion; and
- The dataset may under represent phosphorus concentration due to sampling technique.
Samples tend to be drawn from the surface of a manure storage, whereas solids containing phosphorus tend to settle toward the bottom. If the storage is not thoroughly agitated prior to sampling, phosphorus may be under represented.
For book-values in Saskatchewan, the average value observed in Saskatchewan of 5.6 g P/10 L will be selected. This falls between the average value observed in Manitoba and the value predicted by the ASAE model.
Manure volume for lactating cows (as-excreted)
The volume of manure produced by a dry cow should be somewhat less than that of a lactating cow less milk house waste. Without the addition of 40 L water, a 669 kg (1,475) cow would have manure production of 75 L/lactating cow/d. In the dataset collected by Nennich et al (2005) the average manure excretion of a 669 kg lactating cow is 70.4 kg. This is equivalent to 72.4 L at moisture content of 88 per cent. This lends confidence to the estimates of both 40 L wash water and 115 L manure production of a lactating cow.
Since dry cows are fed less than lactating cows, manure production of a dry cow should be somewhat less than 75 L/cow/d.