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Does shearing increase lamb growth rates?

  • By: "Prime" Ag News
  • Jan 17, 2018

Many of you would have been advised by your agent, neighbour and traders, to ‘knock the wool off’ your lambs to increase growth rates. It’s a question that arises at BCG sheep events, and while the answer from the presenter is always the same, there can be mixed responses from the audience.

So what does the science say?
“The balance of scientific evidence suggests there is no guaranteed effect on lamb growth”

(New Zealand Ag Research Review, 2012)
Professor Bruce Allworth presented a Making More from Sheep webinar in November, beginning with this conclusion and indicated that:
    * Occasionally there is an increase in growth rate
    * But it doesn’t happen often
    * And it’s not guaranteed.

You may shear lambs hoping for an increase in growth rates, but there are often management reasons that are far more important such as grass seeds, fly control and contract requirements.

Effects of shearing
There’s an increase in feed intake of around 40% after shearing lambs, the amount depending on the weather conditions post shearing – greater when it’s colder (up to 70%), and lower increases in milder weather. This response is sustained for four to six weeks before returning to pre-shearing intake levels.

Sheep will spend less time grazing after shearing, but will take in more mouthfuls per hour to increase their intake. However, this is not to make us more money! Rather, it leads to a less efficient conversion of feed into body weight – the extra feed intake is for thermoregulation, ie. to use energy to keep warm by shivering, or pant to keep cool – and that energy can’t be used for growth. Therefore we don’t see the corresponding response in weight gain that we might expect.

If you have shorn sheep in winter (when feed is lower) you may have noticed that they either lose weight, or you need to stock them lightly because they will eat more in that month after shearing. Sheep that are shorn in summer that are not provided with shade, will have trouble with heat on hot days because wool is an insulator.

Increased feed intake means they are eating more and there is more gut fill. Sheep may appear fuller, and if you weigh your sheep two to three weeks post shearing it appears they’ve put on weight, particularly if there’s excess green feed, but it is really just increased gut fill.

When sheep are locked up and held off feed, there will be a depression of appetite. First they’re off feed, so won’t be putting on that weight for that period, but there’s a second effect on appetite that kicks in after about 6-12 hours – as sheep are held off feed for an extended period of time, the time it takes for them to regain their normal appetite increases. If returned to feed after 12 hours they’ll usually resume normal appetite straight away, but if held off for 24 hours, it may take them at least another 24 hours before they regain their full appetite. In this case you have two days of depressed growth rates to consider if you are trying to finish lambs.

Of seven research trials conducted in Australia, New Zealand and the UK, one trial in 1966 had a positive growth rate response to shearing, but 6 trials since then (the most recent in 2015) found little to no difference.

Professor Allworth also noted:
   * Crossbred and Merino lambs have the same physiological response to shearing
   * Cover combs have no benefit, ie cause the same response as traditional shearing
   * Shearing can decrease fat cover, as there is some mobilisation of fat for thermoregulation. Consider how long after shearing you will be selling the lambs. Shearing has improved carcase yield.
   * There has been no growth response of shearing lambs entering feedlots. In fact, the lambs ate more purchased feed, so shearing them cost more. If feed is surplus in the paddock this cost isn’t noticed as much.

When is shearing likely to be beneficial?
If you need to shear you must consider the likely weather conditions and the feed available. The best scenario is to have excess feed available (no extra cost for the extra intake, ie. good pasture or Lucerne) and mild weather for about a four to five week period, which is hard to guarantee. Looking across trial results this is the most logical time for a response, but trials also indicate even under these conditions you may not get the response, and having lambs locked up prior to shearing may counteract any gains that occur.

If you don’t shear, you may need to:
* Watch grass seeds – try not to compromise skin quality
* Treat for flies – cryomazine (Vetrazin™) or dicyclanil (CLiK™)
* Decision on whether to shear lambs?

Professor Allworth stressed that you shouldn’t base your decision to shear on possible weight gains.

    * Evidence is that there will be little or no growth rate gain – it’s a small chance only if there is excess feed and mild conditions, but not guaranteed.
    * By the same token, shearing won’t disadvantage lamb growth rates either.

But base your decision to shear lambs on:
   * Cost of shearing: $4-$7/hd (including shed costs, may be less if there’s an alternative to crutch)
   * Value of wool / penalty for skin value
   * Cost of fly control
   * Ease of management, eg. feedlot dust
   * Access to market (contract specification, appearance).