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What should be considered when adjusting to spring pasture

Dairy Pipeline: April 2000

Charles C. Stallings
Extension Dairy Scientist, Nutrition
Virginia Tech
(540) 231-4758

This time of year the new grass can supply an excellent source of nutrients for heifers, dry cows, and lactating cows. Pasture is succulent and readily digested. Usually milk is higher after going to pasture but milk fat percent is reduced. The question always comes up, What should I change in the supplement?

A study published in the Journal of Dairy Science (82:153) from the University of Minnesota looked at different types of energy sources added to a pasture base (50% orchardgrass, 25% alfalfa, and 25% red clover) in a continuous culture fermenter in order to stimulate rumen conditions. Diets contained 99.5% pasture and .5% mineral or 55% pasture, 45% supplement, and .5% mineral. Supplements were either beet pulp, cracked corn, or soybean hulls. All supplements resulted in a reduction in rumen pH. Corn and soybean hulls increased the total volatile fatty acid produced indicating they stimulated fermentation. Corn resulted in a reduction in the acetate to propionate ratio compared to soybean hulls or pasture alone. Both corn and soybean hulls resulted in more bacterial nitrogen production. It seems this improved bacterial flow is the main advantage to supplementing pasture fed cows.

Pasture has a lot of soluble and rumen degradable protein. That means rumen available energy from nonstructural carbohydrates (grains) and digestible fiber (soybean hulls) are needed to transform the ammonia nitrogen into bacterial protein. Physical form and type of grain will have an impact on degree and rapidity of fermentation and formation of bacterial protein. Finer ground grains will be faster in fermenting. Also high moisture grains are faster degrading in the rumen as is barley relative to corn. The maximum rate of supplementation should be evaluated considering these factors.

Work by Dr. Carl Polan here at Virginia Tech indicates that adding more than 16 lbs. of corn per cow usually shows little advantage when considering increased milk production and 12 lbs. may result in as much milk as 16 lbs. However, all of this is very dependent on high quality forage managed intensively so that abundant quality feed is available.

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