Forage Quality for Dairy Cows: Corn Silage, Alfalfa and Forage Testing

Walk through a dairy feed ration and the most important line on the page is usually not the grain, the protein supplement or the vitamin mix. It is the forage. Corn silage, alfalfa haylage, grass hay and other forages typically make up more than half of a dairy cow's diet, and their quality sets the ceiling on how much milk the herd can produce from home-grown feed. Two loads of silage that look the same can differ widely in energy, fiber and digestibility, and the cows will tell you the difference at the bulk tank.

This guide explains how dairy nutritionists think about forage quality. Numbers and recommendations vary by region, forage type and herd, so work with your nutritionist and your local extension office. If you want to find forage and dairy nutrition workshops near you, agriculture.directory can help you find agriculture events in your area.

Why Forage Matters

A cow's rumen needs fiber to function, and forage provides both fiber and a large share of her energy and protein. Better forage allows more of the ration to come from home-grown feed and less from purchased concentrates, which lowers feed costs. Poor forage forces a farm to buy more supplements to make up the difference. University dairy programs such as University of Wisconsin Extension, University of Minnesota Extension dairy and Penn State Extension dairy publish detailed forage guidance.

Corn Silage

Corn silage is a major energy source in many dairy rations. Quality depends on the hybrid, harvest timing, chop length, kernel processing and how well the silage is packed and covered. Corn silage is usually chopped when the whole plant is in a certain moisture range, often cited as roughly 30 to 35 percent dry matter, though recommendations differ by storage type. Chopping too wet causes seepage and poor fermentation, and chopping too dry makes it hard to pack and raises the risk of spoilage. Kernel processing breaks open kernels so cows can digest the starch.

Alfalfa and Other Legume Forages

Alfalfa is a high-protein, high-fiber forage that is widely used in dairy rations. Its quality depends largely on the stage of maturity at cutting, since alfalfa cut earlier is higher in protein and more digestible, and later cuttings have more fiber and lower quality. Weather at harvest also matters, since rain on cut hay leaches nutrients and delays drying. Many herds make alfalfa into haylage or baleage, which is wrapped or stored wet, to speed harvest and reduce field losses.

Grass and Mixed Forages

Grass forages, hay and pasture play a role in many herds, particularly smaller and grass-based dairies. Quality is again driven by maturity, and young, leafy grass is far more digestible than mature, stemmy grass. Mixed stands of legumes and grass offer a balance.

Key Quality Measures

• Dry matter. The portion of the forage that is not water. It affects how you calculate feeding amounts and storage success.
• Crude protein. The protein content, important in alfalfa.
• Fiber measures. Neutral detergent fiber, or NDF, and acid detergent fiber, or ADF, describe the cell wall content. Higher fiber generally means lower energy and lower intake.
• Digestibility. NDF digestibility estimates how much of the fiber the cow can digest, and is a strong predictor of milk potential.
• Starch. Important for corn silage energy.
• Fermentation analysis. For silages, indicates pH, acids and how well the forage fermented.
• Mold and mycotoxins. Moldy forage can lower intake and harm cow health.

Forage Testing

A forage test sends a sample to a laboratory and returns a report listing measures such as those above. It is one of the cheapest and most useful tools on a dairy. To get a good sample, collect from several places in the pile or bale lot, mix well and send promptly, following the laboratory's instructions. Test each new field, cutting and storage unit, and retest when forage changes. Nutritionists use the numbers to balance rations, and changes show up quickly in milk and components.

Particle Size

How long the pieces of forage are matters for rumen health and chewing. Particles that are too short can reduce rumination and raise the risk of acidosis, and too long can lead to sorting. Many nutritionists use a particle separator, such as the Penn State Particle Separator, to check ration particle size. Extension programs offer guidance on targets.

Harvest Timing and Weather

Quality decreases as plants mature and with every day cut forage sits in the rain. Planning harvest around the weather and crop stage is a major part of forage management. Many herds hire custom harvesters for timeliness, and others invest in equipment to speed harvest.

Storage and Shrink

Storage losses are an often hidden cost. Silage and baleage lose quality if oxygen enters, so the key practices are packing tightly, covering quickly and sealing well, and keeping a tight, clean feed-out face with enough removal each day to avoid heating and spoilage. Hay stored outside loses more than hay stored under cover. Track inventory so you know how much forage you have and when you will run short.

Feed Inventory and Budget

Estimate the amount of forage in storage and compare it to what the herd will eat. A shortage forces expensive purchases, and a surplus is wasted money. Our dairy budget guide on this site discusses feed costs. The Merck Veterinary Manual covers dairy nutrition and metabolic disease linked to poor forage or ration problems.

Common Mistakes

• Not testing forage. Numbers beat guesses.
• Harvesting too late. Quality drops fast.
• Poor packing and covering. Oxygen ruins silage.
• Ignoring particle size. It affects rumen function.
• Not tracking inventory. Shortages cost money.

Farm.fm's page on a dairy nutrition expert who gives a cow a personalized diet is a humor piece on dairy feeding, and for another, see farm.fm's piece on dairy cows demanding better working conditions.