In addition to athe nnouncement that it was the season to donate shekalim, rosh chodesh adar was the time bais din had to send out warnings about kilayim. The Bartenura explains that seed can contain a certain percentage of mixture, e.g. if one has wheat to plant and there is a few kernels of barley seed mixed in, that would not be a problem, provided it is below the threshold of 1/24. (The Yerushalmi yomi we just past this exact sugya.) The issur here is not planting kilayim, because one wishes to plant a wheat field and does not want the barley; however, to avoid mar'is ayin of any mixture, the chachamim set a cap on the impurity of the seed.
If one actually planted a field mixing different seed and violated the issur of kilayim, then it is not sufficient to pluck out a few of the mixed plants below a certain threshold, but the entire field becomes assur.
The Rishonim ask: why by the mixture of seed do I assume if the seed is below a certain percentage it is bateil, but if I intentionally create a mixture of seed and plant it in my field, the entire field becomes assur and we do not say the minority crop is bateil to the majority of the field?
The answer is that by other issurim, e.g. chazir, the Torah prohibited a k'zayis of the item; by definition, once that item is in a ta'aroves and is bateil, it not longer exists. By kilayim, by definition the issur is the mixture, not each individual item. In normal cases, bittul is a matir; by kilayim, bittul is the oseir.
If that is so, then it follows that bittul should never work by basar b'chalav, because there too, the issur is not the individual meat or milk, but the ta'aroves itself is the oseir. Why then is there a halacha of bittul b'rov by basar b'chalav?
The Ran answers that "derech bishul asra Torah". The reason that basar b'chalav is permitted if the % in a ta'aroves is negligable is not because of the normal din of bittul b'rov, but because cooking such a small amount is not called bishul.
My question: if so, why can't I intentionally put a drop of milk in my cholent? By other issurim, I would say "ain mevatlin issur l'chatchila", that bittul can only be used ex post facto after the ta'aroves happens, but by basar b'chalav, there is no halacha of bittul - there is just a din that defines the parameters of bishul and teaches me that cooking a little drop is not called cooking?!