Dry Matter Yield

Forage is bought, budgeted and fed on dry matter, because the water in a wet crop is worth nothing and weighs a great deal. Forty-five tonnes per hectare of fresh maize silage at 32% dry matter is $45 \times 0.32 = 14.4$ t DM/ha, and the other 30.6 t is water that still has to be chopped, carted, ensiled, stored and eventually pushed up to the feed fence.

The reason this conversion is not optional is that fresh weights are not comparable to one another. Two fields yielding 45 t/ha fresh, one at 28% DM and one at 35%, differ by a quarter in what they actually produced — 12.6 against 15.8 t DM/ha — and no ranking of varieties, cutting dates or fertiliser treatments means anything until both are on a dry matter basis. The same applies to buying standing crop by the acre and to any comparison across cuts, since a first cut of grass and a rain-delayed second cut are rarely within ten points of each other.

Dry matter content is also an agronomic target in its own right, not merely a conversion factor. Maize is ensiled at 30–35% DM and grass silage at 25–35%. Below about 25%, a clamp produces effluent — an environmental liability as well as a loss — and ferments toward butyric acid instead of lactic, which damages palatability and intake. Above about 40% the crop is difficult to consolidate, air stays in the clamp, and the loss shifts from fermentation to moulding. Hay is a different regime entirely, baled at 85% DM or above, below which it heats and can reach ignition.

Measuring it is easy and routinely skipped. A kitchen scale and a microwave give a usable field figure in twenty minutes: weigh a sample, dry it in short bursts with a cup of water in the oven to prevent scorching, and reweigh until the mass stops falling. The laboratory method is 105 °C to constant weight, and the classic mistake with either is confusing dry matter with moisture — they are complements, so 32% DM is 68% moisture, and entering the wrong one here is a factor of two in the wrong direction.