Labs can do leaf testing from plants still in veg. I have only heard of this being done when looking high CBD phenos. This is in no way an accurate way to determine what cannabinoid levels of finished flowers will be because total cannabinoid levels in leaves are usually in the 0.5% total or less range. This testing comes in handy when doing CBD pheno hunting by seed, any measurable amount of CBD in leaf material usually means you will yield high CBD flowers.
Would just like to add what should also be noted
is the ratio of thc:cbd in the results of this leaf testing.
I test with Iron Labs for a variety of reasons. I like to analyze data. When I can see my results and compare them with all of the public results going back 3 years, a lot can be learned, heck just looking at the pics and seeing the broad spectrum of plants, ie. comparing all the gdp samples and see how different they look, because they are different. My Mosca Seeds Cinderella 99 BX-1 tested higher in THC than all other Cindys with public results tested there, that doesn't mean anything other than just that, it certainly wasn't the best looking sample they have up. My Star Tonic had the 2nd highest THC% of all Star Tonic's tested at that time, but had the lowest CBD% of all the ones tested. Now hmmmm, that tells me something.
Just wish they would attempt at gathering as much data as possible like: Breeder, Clone or Seed, Flowering Days, Dry Days, Cure Days and that all tests included terpene profiles. One day I envision being able to look at a terpene profile result of a sample and like Neo looking at the Matrix we will be able to know what that sample smells and tastes like because we have an understanding of the data. Their site could certainly be better at making comparisons than the regular spreadsheet options, and it might not be worthy now, but the more data compiled, the more ways to better extrapolate.
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