The Pareto principle (also known as the 80/20 rule) states that, for many events, roughly 80% of the effects come from 20% of the causes. It is comforting to see sheep parasites try to also keep to this rule.
We recently took fecal samples from all the lambs and sent it away to have fecal egg counts done. We are doing this for NSIP and our breeding program. With more lambs to work with this year we also have more data and with more data we can see a clearer picture.
We all know that sheep have parasites and when they graze on pasture they contaminate the pasture and increase the parasite load. Evasive grazing helps but is not always a realistic option, we do FAMACHA but that only tells you if an animal is anemic or not (might be resilient and is shedding large amounts of parasite eggs). What if you could reduce the parasite load by 80% without deworming (I know, sounds like a late night infomercial)
We asked ourselves, how many lambs would have to be culled in order to reduce the parasite load by 80%? The short answer, about 37%. Yes, 37% of the lambs were responsible for 80% of the parasite eggs that are being shed on the pasture. This was not totally unexpected, we have encountered literature where the expected number is that about 20% of animals are responsible for 80% of the parasite load, it just means so much more when you experience it yourself.
The chart below says it all, you can click on the image to open a PDF that you can zoom into to see the detail. The y axis shows the % of eggs that are being shed while the x axis shows the % of lambs. Following the 80% mark on the y axis you can see that it intersects at about the 37% mark on the x axis. No doubt, selecting for parasite resistance makes a difference.