Jim Moseley, Dairy Nutritionist
I’ve been surprised over the past few weeks by the number of requests to update dairy diets due to the addition of pasture. Although many of our customers feed total mixed rations (TMR), even these are being supplemented with pasture. The changes create challenges, but if properly managed, present opportunity for economic gain for the dairy operation.
Well-managed pasture provides the dairy producer with low-cost, high-quality forage. In the northeast, there is an increasing number of farms where pasture provides a significant percentage of the forage during the grazing season. An effective grazing program requires about one acre of pasture per animal, continuous rotation through small paddocks, free-choice water in each paddock, and supplemental grain. The optimal nutrient value of pasture grass is reached when the plant is 6 to 8 inches tall. At this, the vegetative stage, the forage will contain high levels of protein (22-28%) and low levels of fiber (20-30% ADF and 45-55% NDF). Cows should be allowed to graze a paddock between milkings then moved to a new paddock after each milking. Paddocks should be sized such that during the 12-hour grazing period cows consume desired grasses to a height similar to what would be left if mechanically clipped. However, diverse pasture is never grazed to a uniform height. Cows will select what they prefer, and if given the opportunity, will overgraze preferred grasses. If not overgrazed, paddocks will recover and be ready to graze again in 10-15 days in the spring and 20-30 days during the summer. If properly rotated and weather permits, quality and quantity can be maintained throughout the entire grazing season. To encourage cows to eat most of the grass, paddock sizes can be adjusted using temporary fencing. During the early season, paddocks are kept small, and then as the season advances, paddocks are enlarged. Rather than letting grass grow past its prime feeding stage, hayable paddocks should be mechanically harvested so they can regrow and be put back into the rotation. The first paddocks to be grazed in the spring should be those that cannot be harvested.
To best take advantage of any forage, the entire diet should be balanced using a computerized ration balancing program — either Solids Solutions or Precision Dairy Nutrition. This ensures that nutrient needs are met and “best cost” grain ingredients are selected. Occasionally, pasture grasses should be analyzed to determine their nutrient content. To accomplish this, cows need to be watched to determine which grasses they prefer. Then collect and freeze a sample of what they are eating and send to the lab. Freezing is necessary if an accurate estimate of the protein fraction is to be made. Samples that are submitted without freezing will overestimate the amount of soluble protein that the cow actually gets while grazing. When entering information into the program it is normally necessary to either set a maximum constraint on the amount of pasture or put a minimum on other forages. Since pasture is so high in protein, if possible, it is a good idea to offer at least 10-25 lb of corn silage. This will add additional energy and dilute the pasture protein with energy that also will provide at least a small amount of fiber. Vegetative grass is high in soluble carbohydrate, much of which is in the form of sugar. It is possible, therefore, to meet total fermentable carbohydrate needs with conservative amounts of starch and non-fiber carbohydrate. The high fermentable-carbohydrate levels complement the also high degradable-protein levels to promote lots of rumen microbial growth. It is not unusual to see as much as 52-55% of the total metabolizable protein coming from the rumen bacteria. Grain mixes formulated to complement these diets generally contain large amounts of corn meal, lots of mineral, and are low in protein. Monitoring milk urea nitrogen (MUN) levels in the bulk milk tank is one way to track protein adequacy of the diet. When cows are on lush pasture, it is common to see MUN levels greater than 15 mg/dl, so balance metabolizable protein accordingly to bring MUN down in the 10-15 range, if possible. To ensure good rumen function in the face of relatively low fiber intakes, it is advisable to feed 4-8 ounces of sodium bicarbonate. Also, to help prevent grass tetany, magnesium oxide should be used to increase dietary magnesium levels to .35- .40% of the dry matter.
Diets containing relatively large amounts of pasture look a lot different than the winter diets that we in the northeast see for so much of the year. Different, yes, but it is important to remember that ruminants by nature are grazers. With a little help from a balanced diet, they can make a lot of high-component milk at relatively low cost. With the state of the dairy economy today, many of our customers are looking to take advantage of the economics. We need to help!