Genetic Selection of Rideau Sheep for Parasite Resistance at Breezy Ridge Farm

Resisting Parasite Infection

Haemonchus contortus is a gastrointestinal nematode responsible for almost all morbidity and mortality in grazing sheep in Ontario. The warming effects of climate change in addition to the parasite’s rapidly developing resistance to anthelmintics means that finding a new, sustainable solution to GIN infection, such as genetic resistance, is very important.

Ontario Lambs Are Not Raised on Grass

In Ontario, lambs are generally raised indoors. However, animals must be on grass long enough to have been “challenged” in order to produce an immune response we can test. Lambs born indoors will not be tested until the following year, and ewes are not put on grass until lambs are weaned. Both situations mean that there are long periods between exposures, which is a problem for our selection protocol.

What is the Selection Process?

Fecal Egg Counts (FECs) are conducted for management groups every few weeks once the animals are on pasture. When the group FEC reaches 800 eggs/gram, individual tests are performed. Individuals are organized into HIGH, MEDIUM, and LOW categories. Animals that are LOW (500 eggs/gram or less) are selected for breeding because they shed fewer eggs onto pasture and are less susceptible to infection.

How are Resistant Rams Selected?

All rams are selected from ewes that are 5 years old. with a 301-day lambing interval and weaned 2.5 lambs per year which are 22.kg or higher at 50 days and with good Gen-Ovis performance. Rams are also Genotyped for both Homozygous resistance to Scrapie by looking for a Q/R or R/R and the Maedi Visna resistance Haplotype gene of 1,1; 1,4 or 4,4. Then all rams are Fecal egg tested twice, 4 weeks apart during the heavy worm season of summer to detect resistance to GIN.

How Can We Look at the Data?

There are still high and low FECs every year, as there will always be variation between animals. Over time, the median/average parasite load has been decreasing. This implies that the selection protocols being used have been working, and that animals are becoming more resistant to infection. Additionally, the lambs born since the selection protocol was started are heavier and there are more born per ewe.

What Has Been Observed on Farm?

Every year since starting this selection process, less money needs to be spent on dewormers, and less time drenching animals. On-farm deaths due to high parasite loads/high parasite susceptibility have been virtually eliminated. Overall, ewes are in better condition, they are milking harder, and have increased fertility.

Challenges to the Selection Protocol

Variable climate influences the time animals spend on pasture. Differences in when animals were put on grass and differences in the period of time they have spent on grass every year contribute to variable FECs. Climate also affects the life cycle of the parasites. Warm, wet weather can increase their metabolic and reproductive activity, while cool dry weather hinders it.

What Comes Next?

In the future, FEC data and the relevant genetic information will be incorporated as part of a selection index in GenOvis. Eventually the goal is to share this selection protocol with other producers in Ontario. Resistance can also be spread by selling more rams with resistant tendencies to increase the distribution of resistant genetics across the province.

This boxplot displays the range of the log transformed FEC by year. The median has been decreasing following the implementation of this selection process in 2014. The large variance seen in 2018 can be explained by a number of factors. First, considerably more samples were taken and for the first time, these included mature ewes. Second, this year involved the latest collection date thus far (October 22nd) which captured more of the late season parasite load than previous years.

Years	# of lambs born	lambs weaned	50d weight (kg)	kg/ewe
2009–2013	2.11	485	21.38	45.11
2014	2.16	509	22.12	47.78
2015	2.48	661	22.14	54.9
2016	2.37	652	21.23	50
2017	2.5	699	22.96	57.4
2018	2.35	526	24.16	56.77
2019	2.55	706	24.1	61.45