Dry Matter Intake from Body Weight

Everything else in a ration is bounded by this number. An animal has a finite appetite, and no amount of careful balancing helps if the ration it produces is larger than the animal can physically eat. So intake is estimated first, and the usual estimate is a percentage of body weight: a dry beef cow around 2%, growing and finishing cattle 2.5–3%, a high-producing dairy cow up to 4%, sheep and goats 3–4%.

The word that carries all the weight here is DRY. A cow does not eat silage, it eats the dry matter in the silage, and the percentage above is dry matter against live weight. Quote an as-fed intake against the same percentage and the ration collapses: 3% of a 600 kg cow is 18 kg of dry matter, which is about 51 kg of silage at 35% dry matter. Feeding 18 kg of silage instead delivers 6.3 kg of dry matter, roughly a third of what the animal needed, and the deficiency shows up as lost condition long before anyone suspects the arithmetic.

The percentage is not a constant of the animal, and treating it as one is the second mistake. Intake falls with the fibre content and the bulk of the ration, because a ruminant on coarse forage is limited by how fast the rumen empties rather than by energy demand. It falls in heat, in the last weeks of gestation when the calf crowds the rumen, and with any illness. It rises with production, with palatability, and with feed processing that speeds passage. Two cows of identical weight on different rations can differ by a third.

Use this figure as a sanity check on a ration rather than as its foundation. If the ration a nutritionist builds calls for more dry matter than 3% of body weight for beef cattle, either the ration needs to be denser or the assumption about what the animal will eat needs revisiting — and the honest way to settle it is to weigh what goes in the bunk and weigh what comes back out of it.