I need to learn what?

Hmm, that sure is a bit of a counterintuitive brain-bender! (Adding up two vertically-separated 屮's [especially when 屮 is a Kangxi radical in itself, number 45] to arrive at 艸 / 艹 [Kangxi number 140] as the radical?!).

From a much more logical, sensible, and ultimately simplified-character point of view, here's what can be said about 芻 (taken from the second of the jpegs posted in my thread entitled 'Guide to Simplified Radicals'):

64 屮 a sprout. Central piercing stroke last. Lone (芻) chú under radical 20 also; simpler 刍 under 30 & 60.

More progressive (i.e. less hidebound and dogmatic) Kangxi-based dictionaries, e.g. the ABC ECCE for one, will also list 芻 under 勹 too (i.e. the CASS radical 20 mentioned just now).

MDBG however is not one of them, because it unfortunately doesn't provide (m)any of these alternative, more logical look-ups.

;)