Managing pasture-based dairy diets can be a real challenge, says Larry Muller.

“Graziers don't have as much control over their feeding programs as non-graziers do,” says Muller, a Penn State University dairy nutritionist. “A big challenge arises for graziers because the amount of dry matter available per day changes as the season progresses and pasture growth slows and quality changes.”

“Flexible management is the key because the weather changes and you're feeding the herd with a forage supply that changes,” adds Dennis Johnson, a University of Minnesota dairy scientist.

Grazing herds should be allocated enough pasture to enable cows to consume large amounts of forage.

“That's the most difficult thing in managing pasture — making sure you allocate enough pasture each day for the number of cows you have,” says Muller. “Producers should think in terms of a pasture being a feedbunk and strive to keep it full to maximize pasture dry matter intake.”

As a guide, at least 50 lbs of forage dry matter should be provided per cow per day, says Muller. Tools like sward sticks and rising plate meters can be used to estimate the dry matter available per acre. Then simple calculations can determine the size of the paddock needed each day.

“If you have 100 cows, you need about 5,000 lbs of available dry matter/day. If one of your tools tells you that the pasture will provide 2,500 lbs of dry matter/acre, then you need to provide 2 acres/day to have 50 lbs of pasture dry matter available/cow/day,” says Muller. “These calculations should be done frequently, as forage availability changes rapidly during the grazing season.”

Monitor your herd's daily milk production, he advises. “A decrease in milk production tells you that maybe there wasn't enough pasture available for those cows to maximize intake,” he says. “If that's the case, you need to give them more pasture or add more supplemental forages, such as hay or corn silage, to the ration.”

“Take your scissor cuttings at the height that's typical of how the animals graze.”
— Dennis Johnson

Once you've determined the forage quantity available, you must know the quality of the pasture. Minnesota's Johnson encourages producers to collect scissor cuttings or grab samples that simulate grazing and send them to a forage-testing lab for analysis. Take samples at several locations throughout the pasture, mix them together and send a smaller subsample to the lab.

“Take your scissor cuttings at the height that's typical of how the animals graze,” says Johnson. “If they don't graze all the way to the ground, don't cut it all the way to the ground.”

While a good mix of pasture species varies by region, in the Midwest Johnson prefers about 45-50% alfalfa plus equal amounts of late-maturing orchardgrass, bromegrass and some kura clover.

Next, formulate a supplement that complements the nutritional profile of the pasture. The supplement you feed in spring should be different from what's fed in summer because pasture quantity and quality change.

“The most limiting nutrient in a pasture-based diet is energy, so look to traditional high-energy grain sources, such as corn, barley or wheat, to fill the gap,” says Muller.

The most common question he gets asked is how much supplement should be fed.

“Generally speaking, early lactation, high-producing cows in the U.S. on pasture can be fed 16-18 lbs/cow/day. The milk response to supplementation and today's favorable milk-feed price ratio make this possible. For the total lactation, an average of 12-15 lbs/cow/day is usually profitable.”

To balance the spring pasture's lower fiber content, feed high-fiber, low-starch byproduct feeds like soy hulls, beet pulp, brewer's grain and/or fuzzy cottonseed.

“Including some of these ingredients and still including about half ground corn in the supplement has given improvements in milkfat test,” says Muller.

“Slowing the rate of digestion helps, too — especially in spring when pastures are highly digestible and wet. Maybe you want to feed 2-3 lbs of long hay, which creates a good fiber mat in the rumen vs. wet forage.”

In summer, when more supplemental energy is needed, Muller suggests bumping up the corn grain to about 80% of the supplement ration.