In some random sleuthing about dates and events and how the trial is being presented, I noticed some corresponding dates.
September 9th, 2013, police return to the Millard farm in Ayr after learning new information that led them there.
September 18th, 2013 is the day the officer tailed CN to obtain cast off DNA.
It lends me to think that the need for her DNA is related to the new information or something discovered during that later farm search.
http://globalnews.ca/news/828217/tim-bosma-toronto-police-once-again-searching-dellen-millards-farm/