How many stock units are ewe really running?

Stock units are a commonly used tool for pastoral agriculture in New Zealand. They are utilised to make comparisons between and within farms, quantify rural lending and valuation, and aid in farm system analysis and design. With so much often relying on the accuracy of the stock unit, it would be fair to assume that it’s a thoroughly defined, easily applicable measurement. However, when it comes to sheep, there are notable discrepancies in its use.

The modern stock unit is equal to a feed requirement of around 6000 MJME/year.* In the paddock, this equates to 520 kgs of dry matter, or 3500 kgs of fresh ryegrass/white clover pasture. This is designed to represent the yearly feed requirement of a single ‘average’ ewe, weighing 55kg and rearing one lamb.

However, the yearly feed requirement of an animal can be affected by a number of variables, including size, level of production, and rate of mobilisation of body reserves. For example, an animal which is producing milk is particularly prone to mobilising its body reserves- this is sometimes referred to as maternal elasticity. The amount of mobilisation that occurs then influences the amount of condition which must be regained before the next production cycle begins. It is easy to see how variation in even one of these factors can result in the feed requirement of an animal moving away from its theoretical value of 6000MJME/year.

The table below summarises a very basic model which shows how changing some of these factors can affect the stock unit value of a particular ewe.

Reproductive rate in all scenarios= 130%. Some scientific studies have supported the idea that there are ewes who are genetically predisposed to mobilising their body reserves to support lactation- they have ‘elastic’ liveweights. This model has represented that by inputting ewes as either ‘mobilising tissue’ or ‘static liveweight’. ‘Mobilising tissue’ implies a loss of 1 unit of body condition score throughout lactation, equalling 8kg liveweight loss. 30 MJME were released per kg of tissue mobilisation; 50 MJME were required per kg tissue deposition. It was assumed all energy from tissue mobilised was utilised for lactation. The increased milk supply resulted in a 5kg increase in weaning weight.

It’s intuitive that larger animals are worth more stock units. This is because their higher liveweights mean they have higher maintenance costs- it takes more fuel to keep a bigger machine running. This is reflected in the table above, where a 65kg (static liveweight) ewe requires 491 MJME more per year than her 55kg peer.

What’s less intuitive is the fact that ewes who mobilise their body reserves were worth fewer stock units than their static liveweight counterparts. In this model, those ‘elastic’ ewes didn’t lose weight year to year- so they had to regain all the weight that they lost to support their lactation. Surely then, the fact that it requires more energy to build those reserves back up (50 MJME per kg tissue) than the amount that is released when those reserves are mobilised (30 MJME per kg tissue) means that elastic sheep have to eat more per year than their static liveweight counterparts at the same bodyweight? In short, the answer is yes. So, there must be another factor having a large enough effect to more than compensate for that.

That factor is the elastic ewe’s lamb. It’s important to remember that a stock unit, by definition, includes the feed requirement of that lamb- right up until it leaves the property. Every summer-dry sheep breeder agrees that the sooner a lamb reaches slaughter weight and is sent off the property, the better. After all, finishing lambs becomes increasingly difficult as the spring feed surplus runs out. The sooner they’re gone, the less they have to be fed- and consequently, the fewer stock units that each of those lambs (and their mothers) account for. The elastic ewes’ ability to mobilise their body reserves means that they are able to produce more milk, and consequently their lambs weigh more at weaning than the lambs of static liveweight ewes. Those lambs also reach slaughter weight sooner than their counterparts, and therefore spend less time eating feed on the property.

Interesting though that is, the largest difference in yearly feed requirement was between the heavy and light ewes, not the elastic and static ones. It’s important to note that, in this model, the same weight of lamb was slaughtered in every scenario. Given that, this model suggests that selecting for a small, low maintenance ewe will increase production efficiency more than selecting for ewes which are predisposed to mobilise body reserves. However, elastic ewes may still be valuable in particular feed supply situations.

The stock unit system as it stands is far from perfect. The variation in liveweight within flocks (let alone between flocks, breeds and districts) could easily match the 10kg variation used in the example here. Hopefully, consideration of these factors will help you calculate your farm’s carrying capacity with slightly more accuracy next time you feel inclined to do so- or at least inspire you to take everyone else’s stock unit claims with a grain of salt!

*megajoules of metabolisable energy