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Perception is reality. This is especially true in today's era of media sound bites and one-sided documentaries. The impact of the dairy industry on the environment has certainly been the subject of many of these reports.
One of our main environmental perception problems is manure—it stinks, and it's the first impression the public has when they get near a dairy farm. And it costs money to manage it.
What we feed can have a real impact on the amount of manure produced as well as its composition.
An average Holstein cow producing 70 lb. of milk/day excretes about 150 lb. of manure (feces plus urine) per day with 12.5% dry matter. The average dry cow produces about 85 lb. of manure per day. These numbers vary widely depending on dry-matter intake (DMI), digestibility of the diet and feedstuffs fed.
It's intuitive that as cows eat more feed, they produce more manure. Digestive efficiency tends to decrease as DMI increases, because feed passes through the digestive tract faster.
Manure output is 2.7 lb. per pound of DMI at 35 to 40 lb. DMI/day. When intakes reach 55 to 60 lb./day, output increases to 3.5 lb. of manure per pound of DMI. Keep in mind that milk production usually increases with higher intake, so the net effect is that higher-producing cows produce more milk per unit of manure output.
Interestingly, cows fed corn silage produce considerably less manure than cows fed hay crop forage. Replacing 10 percentage units of hay crop forage with corn silage (as a percentage of forage) can reduce manure output by 4 lb./cow/day. The reason for this difference may be the higher potassium content of hay crop forages. Higher potassium intakes result in increased urine excretion. So it also stands to reason that reducing dietary potassium will decrease manure output.
A lactating cow will produce about 2 lb. more manure per day for every one-percentage-unit increase in dietary protein. But since the range in protein concentration of typical diets is narrow (14% to 18%), there's not as much potential to influence manure output by reducing protein. However, total nitrogen excreted is significantly increased when more protein is fed.
Dietary fiber content and digestibility also influence manure output. In general, a one-percentage-unit increase in neutral detergent fiber (NDF) concentration will increase manure output by 0.5 lb./day to 1 lb./day. Lower NDF digestibility increases manure output. Producers should make sure to harvest forages at immature stages in order to maximize digestibility. Heat stress reduces diet digestibility, so comfortable cows produce less manure per pound of DMI.