Happy dairy month!
As you all probably know June is dairy
month and that means it’s time for a dairy manure story around here. As I got
to thinking about what to discuss - I was working on a manure budget for the
state of Iowa (how much manure to do we have, how much manure can we use, and
how has this changed with time) and I started pondering about all the changes
to agriculture and what it means for manure. Things like moving away from
pasture and to confinement operations, adding storages, injecting manure when
it is land applied all help us collect a larger fraction of the manure produced
and better use it in our crop production systems. However, there is a deeper
question, and that is, how has the amount of manure we got from a cow changed?
There are lots of things that could go into this, the
production system we use, whether we scrape or flush manure, how much if
anytime the cows spend on pasture or a loafing lot, but we are going to take a
look at is how much manure the cow would excrete. Using the ASABE Manure
Production standard (ASAE D384.2). To get some estimates of milk production the
average annual milk production, in terms of pounds per had was obtained from
the Survey of Iowa Agriculture for years 1924 through 2016. Over this time
period milk production per cow has increased by almost 500% (figure 1); at the
same time manure production per cow increased, but only by an estimated 40%
(figure 2). This means that the we went getting about 0.1 pounds of milk for every
pound of manure to more than 0.5 a pound of milk for every pound of manure,
that’s a significant change.
Figure 1. average
milk production per cow per year in Iowa.
Figure 2. Average
manure production per cow per year in Iowa.
However, we can take a deeper a look at what some of these
numbers mean for manure. For example, most of the weight of manure is water –
but what about solids, nitrogen and phosphorus? Again we became more efficient
here, with cows decreasing the amount of solids, nitrogen and phosphorus they
excrete per pound of milk produced by 70-80%.
The actual excretion of solids, nitrogen, and phosphorus did increase,
but only by 50% for solids, 16% for nitrogen, and 45% for phosphorus. If we do
just a little more math and make some estimate about how much nitrogen is lost
during storage, land application, and the availability of the nitrogen, we can
make some estimates about how the manure’s nutrient content has changed (figure
3).
Figure 3. Estimated
manure nutrient content.
While not entirely accurate as in the 1920’s through the
1960’s slurry manure systems were minimally used as animals were kept on
pasture or raised in barns with bedding material and manure was handled as a
solid. However, this work does provide some useful insight to understanding how
this industry is changing, with animals that are much more efficient than they
used to be, making more milk for every unit of manure we now manage. So next
time you have a glass of refreshing milk, take a moment to ponder what the next
100 years will bring and what that could mean for how we manage manure.
