One stock unit is 6,000 MJ of metabolisable energy a year — about 16.4MJ a day, or what a 55 kg ewe rearing a single lamb eats. Every figure below is computed from that animal's actual energy requirement, so it changes with liveweight, pregnancy and lactation instead of sitting still in a table.
These are daily rates, not the published class ratings
A published rating — a breeding ewe at 1.0 SU, a dairy cow at 8.5 — is an annual average that already has her pregnancy and lactation folded in. These figures are for one day in one state, so the same ewe reads 0.72 dry, 1.28 heavy in lamb and 1.62 milking — and over two at her peak. Every one is right about the day it describes. Weight them by how long she spends in each state across a year and you land back near the published 1.0. ⚠️ Which also means you cannot divide a stock-units-per-hectare figure by one of these: those are annual ratings, and these are days.
Ewes
at 75 kg liveweight
Stock units and daily energy for ewes
State
Energy
Stock units
Dry, holding weightNo growth, no milk, no pregnancy
New Zealand has two definitions in circulation and they are close but not the same. The revised stock unit is energy — 6,000 MJ ME a year, which is the basis used here because it is what an energy model can compute. The older classic stock unit describes the same ewe as eating about 550 kg of dry matter a year — the same animal, at roughly 11 MJ per kg DM. Figures on the two bases agree to within a percent or two; they are not interchangeable in principle, so it is worth knowing which one you are reading.
Everything here assumes rolling country and no cold stress, at a diet of 11 MJ ME/kg DM. Steeper country costs more energy for the same production, which means more stock units for the same animal. The deer rows also assume red deer in spring — deer intake and growth swing hard with daylength, and a wapiti or wapiti-cross is a much larger animal.
Then there's your farm
Stock units are for comparing and adding up mobs. What they can't tell you is whether the feed in front of an animal actually gets there — that needs the diet, and it's what our free calculator does.