FORMULA SHELF
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The arithmetic behind the work: what to measure, what to type in, and what the answer comes out as. Every formula says what the symbols mean and gives a worked example, so you can check it against a job you have already done.
SHELVES
Livestock
20 formulas
The single number that tells you if a ration or a system is working. Weigh the animals, do it again in 30 days, and divide.
A lamb goes from 30 kg to 42 kg in 60 days = 0.2 kg/day.
How much feed you get back for every kilo you put in. A higher number means more feed per kilo of meat, and a bigger feed bill.
A calf eats 8 kg of feed to gain 2 kg: FCR 4:1.
The fair way to compare a flock, a herd and a paddock. It also tells you whether you are about to overgraze.
100 ewe lambs at 0.1 LU each on 5 ha = 2 LU/ha.
To size a ration. Get this wrong in either direction and the animal is either hungry or overfed, and you will not see which from the ration sheet.
A 500 kg cow at 3.5% needs 17.5 kg of dry matter a day.
Multiply the animal's liveweight by the ration percentage, then divide by 100. A 600 kg cow on a 2.5% ration needs 15 kg of feed a day. Split between forage and concentrate as the ration sheet says.
600 kg cow x 2.5% / 100 = 15 kg feed per day.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Measure around the chest just behind the front legs (heart girth) and from the point of shoulder to the pin bone (body length), both in centimetres. Square the girth, multiply by length, divide by 300. Best for cattle; less accurate for very fat or very thin stock.
Girth 190 cm x length 150 cm: 190 x 190 x 150 / 300 = 1,805 kg estimate.
Convenience only. Double-check the result against the worked example or a second method before you use it.
A condition score tells you more than a weight, and it takes ten seconds. Handle the animal and look at the hooks, the hips, the tailhead and the ribs.
A cow scoring 3.0 can carry about 550 kg, and loses condition on the way to calving.
Measure it, and aim to get it under 380. A long interval usually means the cow is losing condition before the bull goes in.
Calved 15 March and 20 March the next year = 370 days.
The price of your stock depends on it. A heavy animal with a poor dressing percentage is not worth much more than a lighter one, and the killing sheet tells you which you have.
A 60 kg lamb with a 26 kg carcase = 43% dressing.
Two different questions. Conception tells you about the bull and the condition score; calving rate tells you about the calving shed and the hard cases.
90 cows served, 80 in calf, 78 calves born: 89% conception, 87% calving rate.
For working out whether a flock of hens is worth keeping. The feed per dozen is the number to watch, and it gets worse in winter when the birds are burning energy to stay warm.
20 hens, 5 eggs a day, 1.1 kg of feed a day: 240 eggs a month, 1.65 kg of feed per dozen.
The practical version of the stocking rate. It tells you when to move the herd on, and how long a paddock lasts.
6,000 kg DM/ha at 60% grazeable with 40 cows at 18 kg DM: 3,600 / 720 = 5 days.
Measure the bale in metres, multiply length x width x height to get cubic metres, then multiply by density. A small square bale is about 100-150 kg/m3; a big round bale is denser, about 200-250 kg/m3.
1.2 x 0.7 x 0.4 m x 130 kg/m3 = about 44 kg per small bale.
Convenience only. Double-check the result against the worked example or a second method before you use it.
For planning how many hives you can keep, how much equipment you need and what the honey is worth. See Keeping Bees.
6 colonies at 25 kg each = 150 kg of honey.
The measure of the whole system - rams, condition, feed, scanning and lambing. It is the number to write down every year.
120 lambs from 100 ewes = 120% lambing.
The number that shows whether the land or the cow is the limiting factor, and the reason a good dairy farmer looks at the forage first.
12,000 kg DM/ha at 9.5 MJ/kg gives 114,000 MJ, which is 36,000 kg of milk a hectare.
The honest measure of the system. Write it down every year; it tells you whether the problem is the housing, the feed or the lambing.
10 lambs dead from 100 born = 10% mortality.
Size the troughs. A trough that is too small for a hot day is the reason a cow stops eating. See Water Troughs And Algae.
A 600 kg lactating cow in summer may drink 100 L a day, so a 200 L trough empties twice.
Write the treatment date and the withdrawal date on the animal or in the book, and check the milk before it goes in the tank. See Vaccinating And Cold Chain.
Treated on 1 June with a 14-day withdrawal: nothing in the tank before 15 June.
For working out the wool income from a flock, and for judging whether the condition of the sheep at shearing was right.
A 70 kg ewe with a 4.5 kg fleece yields 6.4% of her liveweight in wool.
SOURCES & PROVENANCE
1 listed
This page is an AI-organised index. The label means the grouping and cross-links were compiled with AI; the information itself comes from the public-domain books, reports and manuals listed below. Open any source to read the local copy or follow the link to the original publication.