March 5, 2010

Breeding sheep to eliminate dags and worms. 7.

By Dr Clive Dalton


These are the last two pathways where emphasis is on the males.

Pathway 3: Selecting males to breed females.
Pathway 4: Selecting males to breed males.

Pathway 3: Selecting males to breed females

  • This will happen by using the two-tooth rams bred from the A team nucleus ewes described in Pathway 2.
  • With all the selection pressure put on their dams, they are bound to be ‘improvers’.
  • They are your ‘power house’ for flock improvement, because a ram has greater genetic influence than any single female.
  • Their mission is to sire as many future females as possible in the flock to spread their genetic potential for resistance to dags and worms.
  • These rams are worth big money and if in doubt, ask yourself where you could buy better ones with intense selection against dags and worms?
  • Also, they have been intensively selected on your farm, so there are no worries about how their progeny will perform.
  • If AI had been a commercial reality, then this would have been the way to use them. But a lot can be done using single-sire natural mating.
  • Past experience has shown that really top rams can be single-sired mated with 400 ewes for one cycle. But using a top ram to mate 100 ewes is normal these days.

Pathway 4: Selecting males to breed males
  • This is the final genetic pathway and comes from using the male lambs produced by the top sires out of the A team ewes.
  • They will go around the loop again described in Pathway 2.

  • Inevitably the top sires of one generation will sire the top sires of the next generation, so a build-up of inbreeding is inevitable.
  • At low levels (e.g. under 7%) this is not a problem, but when it reaches high levels quickly (> 25%) then problems can be seen such as minor genetic defects like undershot jaw.
  • When levels get really high (> 50%) they can be major from ‘inbreeding depression’ where fertility and survival are reduced.
  • In this programme, a top two-tooth ram which finally comes through, when used on the A team ewes could have a dam in there, and a twin sister.
  • He could also have a number of half sisters (by his sire out of different dams) which he could also mate.
  • However, if this bit of intense inbreeding did cause problems in this early stage, the intense selection and culling programme would identify and eliminate them.
  • After a couple of generations of the programme, if there was cause for concern, then one single outcross to rams from a breeder with similar objectives would immediately restore genetic variation to continue making progress.
  • Another method for the long term is to divide the A team nucleus into small sub-flocks (on paper and with different coloured tags) and keep moving rams around these in a planned rotation each year.

Final points to ponder

  • There is no need to panic!
  • You certainly cannot go backwards genetically as everything you have done so far is positive! Sheep that don’t produce dags and don’t have worms are ‘efficient converters’ of their feed so are profitable sheep.
  • What you have done in your flock should be of great interest to your former ram suppliers, as now you have given them a challenge to come up with genetics that will reduce your animal health costs and increase ‘easy care’.
  • Invite them to visit your flock and show them the A team ewes and your sires bred from them. They can go home and plan how to keep your custom in future.
  • The wise ones won’t write you off, as you could have genes to help them in future!
  • For any vets complaining about your low drench purchases, suggest that if they want to keep on selling drench, they should provide a free dagging service with each container. Not many vets would ever have done a week's dagging!

What to do if you hit a worm crisis?
  • This could happen of course. An example would be an unexpected outbreak of Barber’s Pole (Haemonchus contortus) worms or some other species depending on where you farm.
  • Again, don’t panic as now when you use an appropriate conventional drench, because your sheep have never been drenched and developed any resistance, the drench will be highly effective.
  • You won’t need to keep on drenching.

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