A History of the Stock Unit System
The Coop ewe equivalent system was developed, according to Coop, as a tool for measuring the present and potential productivity of farms (Coop 1965). Coop observed that whereas productivity could be readily measured on dairy farms that were essentially single product, this task was more complex on sheep and beef properties. The ewe equivalent system allows for estimates to be made of the feed that is consumed by different classes of sheep and cattle to meet their energy requirements, and for these estimates to be converted to a common base and thereby summed. Although not a direct measure of what animals actually produce, it provides a basis for comparing between farms and between animal groups the amount of consumed feed.
The Coop system was based on estimates of digestible organic matter (DOM) requirements. He calculated that the standard ewe required 370kg of DOM. Based on an assumption of 62% digestibility, this converted to 595kg dry matter.
Coop emphasised that his figures should be taken as approximations. The empirical data came mainly from penned animals and there was considerable uncertainty as to the allowances that should be made for the energy requirements associated both with grazing and also the climate of the external grazing environment. In the case of cattle there were also conflicting empirical data to contend with.
Coop observed that the most important source of error on particular farms was likely to be a failure to take account of ewe liveweight. He suggested that ewes on hard North Island hill country and South Island tussock country weighed 36-45kg, whereas Southland Romneys weighed about 64 kg. He noted that ‘unless liveweight is taken into account quite misleading figures could be obtained’ (Coop, 1965:18).
Coop’s recommended figures are shown in Table 1.
In a commentary written some two years after his original review Coop set out the meaning of a ewe equivalent figure and the potential uses of the ewe equivalent system (Coop 1967). He said (p2):
“It is certainly not a production figure, it is not the carrying capacity of the farm and it is not a measure of grass grown on the farm. It is the number of stock, converted to EE, which the farmer has currently decided to carry on the farm”.
He suggested that the ewe equivalent (or stock unit) system could be used with caution for the following purposes:
i) comparison of current stocking rate on different farms;
ii) as an expression of potential carrying capacity on different farms, soil types or districts;
iii) as a target figure to be aimed at on any particular farm;
iv) as a rough guide to the relative carrying capacity of beef cattle versus sheep, or wethers versus ewes;
v) as a figure to be taken into account in land valuation, i.e. sale or purchase price in terms of so much per ewe equivalent.
Table 1. Ewe Equivalents (Stock Units) as Recommended by Coop
| Stock Class | Liveweight (kg) | EE (SU) |
|---|---|---|
| Ewe |
36 ( lambing 70% ) |
0.8 |
| Ewe |
45 (lambing 90%) |
0.9 |
| Ewe | 54 ( lambing 100%) |
1.0 |
| Ewe | 64 ( lambing 120%) | 1.1 |
| Wether | 36-41 | 0.6 |
| Wether | 50-54 | 0.7 |
| Hogget | 23-41 | 0.6 |
| Ram | 73 | 0.8 |
| Jersey cow | 360 | 6.5 |
| Friesian cow (town supply) | 550 | 8.5 |
| Beef breeding cow | 450 | 6 |
| Weaner cattle | 135-270 | 3.5 |
| Yearling cattle | 270-360 | 4 |
| Two-year-old cattle | 360-450 | 4.5 |
| Weaner cattle | 160-340 | 4 |
| Yearling cattle | 340-500 | 5 |
Since that time modifications to the stock unit system could be described as haphazard in that various users have modified the stock unit values but without any overall agreement by users of the system as to what changes should be made. Also, there has seldom been any explanation of the basis on which modifications have been made. As a consequence there are now several sets of stock unit values used in New Zealand, each being similar in that they are based around the concept of a ewe equivalent, and each described as ‘the stock unit system’. Although these variant systems provide revised empirical values, the scientific foundations of these systems are not explicit.
One widely used stock unit system is the figures reported in Cornforth and Sinclair (1984) using data attributed to J.D.G. Scott and J.L. Adams (Table 2 and Table 3). These figures were also reprinted in Donaldson (1987) and in various issues of the Lincoln University Farm Technical Manual.
The key advance with the figures published in Cornforth and Sinclair (1984) was that explicit allowance was made for the effect on the stock unit requirement of differing animal performance in animal classes other than the ewe. These include hoggets (ranging from 0.6 to 1.2), beef cows (3.7 to 6.3), and beef heifers (4.5 to 6). Figures for deer were also provided for the first time. The authors recommended that where lambing performance was known but liveweight of ewes unknown then the underlined figures in Table 3 should be used.
The second important revision was that undertaken by the Meat and Wool Board’s Economic Service and MAF in 1992 (Table 4).
A considerable number of farm management consultants use the Cornforth and Sinclair figures for individual farm level analyses, whereas at the national level the MWI Economic Service and MAF use the revised 1992 figures. However there are also other variants in use, including figures used originally as part of the Livestock Incentive Scheme in the early 1980s, and adjustments by individual consultants to align the system to their own empirical experiences.
Table 2. Factors for converting stock class to standard stock units as in Cornforth and Sinclair (1984)
| Stock Type | Factor |
| Wether Rams Hogget 30 kg, slow growth rate 40 kg, medium growth rate 50 kg, rapid growth rate (pre-winter hogget weights) Beef cow 350 kg, 68% calves weaned 400 kg, 83% calves weaned 450 kg, 88% calves weaned 500 kg, 90% calves weaned Beef weaners 135-270 kg Beef, 200-400 kg, slow growing 200-465 kg, rapid growing 350-500 kg. Bulls 500 kg Jersey yearling 0-12 months Friesian yearling 0-12 months Jersey heifer Friesian heifer Red deer stags Weaning 15 months 15-27 months adult Red deer females Weaning to 15 months 15-27 months adults Fallow deer – use half of red deer values |
0.7 1.0 0.7 1.0 1.2 3.7 4.4 5.3 6.3 3.5 3.7 4.6 4.7 6.0 1.7 1.9 3.0 3.4 1.4 1.8 2.1 1.2 1.8 1.9 |
Table 3. Factors for converting to standard stock units based on ewe weight and percent lambs weaned as published in Cornforth and Sinclair (1984)
| Ewe weight at mating (kg) |
Percent lambs weaned | ||||||||
| 70 | 80 | 90 | 100 | 110 | 120 | 130 | 140 | 150 | |
| 35 40 45 50 55 60 65 70 75 |
0.65 0.70 0.75 0.85 |
0.70 0.75 0.80 0.85 0.90 |
0.75 0.80 0.85 0.90 0.95 1.00 |
0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 |
0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 |
1.05 1.10 1.15 1.20 1.25 1.30 |
1.10 1.15 1.20 1.25 1.30 1.35 |
1.15 1.20 1.25 1.30 1.35 1.40 |
1.25 1.30 1.35 1.40 1.45 |
Table 4. Stock Unit factors used at the national level by MAF and the MWI Economic Service since 1992
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