Shibbolet shu'al (a grain of the 5 grains of israel) is commonly translated as oats but according to some opinions that is not the case. What do the other opinions believe it is and wherein lies the contradiction?
I like this article's explanation as he delves into both the Modern Hebrew as well as Talmudic explanations. Excerpts:
As described in the Mishna (Hallah 1:1), the five grains are:
chitim, seorim, kusmin, shibolet shual, shifon
Chitim are wheat and seorim are barley. We'll discuss kusmin in a future post, but now I'd like to deal with shibolet shual and shifon (or shipon, more on that in a bit.) In modern Hebrew, shibolet shual means "oats" (although "kvaker" is the common term), and shifon means "rye". This goes as far back as Rashi, who identifies them as such in his commentary (Pesachim 35a). However, many modern scholars disagree with this approach, such as Immanuel Low, who writes that shibolet shual is millet, and shifon is oats (both Jastrow and Ben Yehuda mention this approach), and more recently Yehuda Feliks, who says that shibolet shual is "two-rowed barley" (compared to the four and six rowed barley for seorim) and shifon is spelt (which in modern Hebrew is kusmin). Feliks gives a number of reasons for his conclusions - primary among them is that oats and rye were not commonly grown in the Land of Israel at the time of the mishna (this post does a good job of summarizing his arguments and those that disagree with him, on the identity of shibolet shu'al).
Most of the proofs are based on biological or historical evidence - but I did find one etymological aspect: while many claim that the name "shibolet shual" comes the spike, shibolet, of plant looking like the tail of a fox, shual, (and we find the phrase "foxtail" with a similar meaning in English), Feliks believes that the name comes from the fact that foxes like eating the softer two-rowed barley more than other grains, in the same way that invei shual ענבי שועל - "blackcurrants" (literally "fox grapes") are so called because foxes enjoy eating them.
I like the explanation in the last paragraph, as I was curious about the use of the word "shu'al" which means "fox".
In the Artscroll Mishnayos on Kilayim 1:1, besides bringing the opinion from Y. Felix in the name of Rabbeinu Nosson Av HaYeshivah that it is two rowed barley, the same author is quoted as mentioning the opinion of the Rambam in the commentary there who says it is wild (desert) barely, the wild progenitor of the two rowed barely.
Artscroll also quotes the Aruch, ערך שבל, who identifies it as either rye or oats.