Forage Planting Decisions: Corn Silage or Alfalfa?

Dr. Bill Weiss, Dairy Nutrition Specialist, The Ohio State University 

There is no 'best' forage for lactating dairy cattle. All forages provide the three nutrients needed in the greatest quantities by cows; energy (NE_L), protein, and fiber (NDF). The nutrient provided by forages that is must difficult to replace is effective fiber. Energy and protein provided by forages can be readily obtained from other feeds (for example, corn grain and soybean meal), but fiber provided by forages must be replaced by other forages or to a lesser extent by whole cottonseed. The amount of forage grown on a farm should be a function of the fiber requirements of that farm. An important assumption made for this article is that both the corn silage and alfalfa silage were harvested at the correct maturity for dairy cows. Corn silage harvested too wet or too dry and alfalfa harvested too mature will reduce milk production and alter the conclusions. In addition, all the following calculations were made assuming a conventional corn hybrid is used. Hybrids with high in vitro NDF digestibility can increase milk production and alter the results (for details on comparing different hybrids see https://dairy.osu.edu/resource/feed/cornsilforweb.pdf).

When making planting decisions, you will not know the actual nutrient composition of the forage and you will not know the actual yield. Therefore, decisions should be based on expected or probable results. If you have previously grown alfalfa and corn silage, you can classify your farm as having below average, average, or above average yield potential for corn and alfalfa. For this discussion, calculations were based on the yields in Table 1. Estimated costs (OSU Enterprise Budgets) are in Table 2. Estimated storage costs were $22/ton of dry matter (DM) for corn silage and $11/ton of DM for alfalfa (assumed two fillings per year for alfalfa).

For this article, I assumed that all the forage fiber required by the cows on a farm would be grown on that farm (e.g., no purchased hay). A Holstein cow with an average milk yield of 70 lb/day will require approximately 3300 lb of forage NDF for a 305-day lactation. Assuming corn silage and alfalfa silage average 44% NDF and using the yield data in Table 1 and assuming shrink was 5% for corn silage and 8% for alfalfa silage, the acres of each forage that would be needed to provide 3300 lb of forage NDF are in Table 3. If available land is quite limited, less land is needed to meet the fiber needs of the herd when corn silage is planted, even at a very low yield. Based on the forage cost in Table 3, at all yield potentials, alfalfa is a more expensive source of fiber than corn silage. However, protein supplementation costs are lower for alfalfa-based diets than corn silage-based diets. Using nutrient composition from NRC and average prices for corn grain, soybean meal, and expellers soybean meal, the concentrate needed for cows fed corn silage as the sole forage will cost about $67 (for 305 days) more than the concentrate needed when alfalfa silage is the sole forage. When the increased cost of the concentrate is added to the cost of the forage, on average, cows fed diets based on corn silage will be cheaper than those fed alfalfa silage. However, the average difference between forages is only 3% (probably within the error of all the assumptions used). Yield had a much greater influence than forage species. The cost difference between low and high yields for corn silage was 13% and for alfalfa it was 28%.

Because of increased risk and because of increased variability in diet composition, diets with a single forage are generally less profitable than diets with two sources of forage. Research data show that milk production is equal when cows are fed diets with various corn silage:alfalfa ratios when diets are balanced correctly. Therefore, gross income should be similar and the decision as to which forage should predominate in a diet is a function of cost. Based on the total cost data in Table 3, the ratio of corn silage to alfalfa varies based on yield potential (Table 4). When land is poorly suited for alfalfa (low yield potential), the predominant forage should be corn silage, independent of the yield potential for corn silage. When land is well-suited for alfalfa (high yield potential), then alfalfa should be the major or predominant forage in the diet.

Table 1. Dry matter yields (tons/acre) used for calculations.

Table 2. Estimated total production, harvesting, and storage costs ($/acre).

Table 3. Acres needed to provide enough forage NDF to meet the requirements of one lactating cow (average production = 70 lb/day) for 305 days and costs based on yields in Table 1.

¹Increased cost of concentrate needed to meet the metabolizable protein requirements for cows fed a diet with corn silage as the sole forage compared with a diet with alfalfa silage as the sole forage.

Table 4. Forage source that should result in lowest feed costs based on expected yield potential for corn silage and alfalfa silage¹.

¹++ Corn = forage should be predominantly corn silage; ++ Alfalfa = forage should be predominantly alfalfa silage; + Corn = forage should be slightly more corn silage than alfalfa; and + Alfalfa = forage should be slightly more alfalfa silage than corn silage.