מילון על ויקרא 19:39

And how is [a koy] neither like a behemah nor like a hayyah?It is forbidden because of kilayim [to yoke it] with either a behemah or a hayyah. One who deeds his son his behemah and his hayyah he has not [thereby] given him the koy. If one says, “I will become a nazirite if this is [not] a hayyah or a behemah”, he is a nazirite. In all other ways it is like a behemah and a hayyah: It requires slaughtering (shehitah) like them both; It carries carrion impurity; And to it applies the law relating to a limb of a living being like them both.

In the case of many other commandments Rabbi Jose sought to lighten the burden of their observance. He ruled that no “Bikurim” (offering of first fruits) need be brought from the lands east of the Jordan because it was not characterized as a land of milk and honey.⁹⁾בכורים פרק א׳ משנה י׳. He established the rule that so long as a person is engaged in the observance of one commandment he is absolved from observing any other commandments at the same time.¹⁰⁾סוכה כ״ו א׳. The verse, “Before an old man you shall rise,” (Lev. 19:32) he explained to mean that only a scholarly person is to be accorded that honor.¹¹⁾קדושין ל״ב ב׳.

And Ramban, may his memory be blessed, wrote about the reason for the commandment in his commentaries (Ramban on Leviticus 19:23) [that it is] to honor God, may He be blessed, from the beginning of all our produce, and not to eat from them until we bring all of the fruits of one year [as] 'praising to the Lord.' And behold, within the first three years, the fruit is not fit to offer, since it is meager; and also because [the tree] does not give taste or a good fragrance into its fruit [then]; also because most trees do not put out fruit at all until the fourth year from their planting - and so we wait for them all. And this commandment is similar to the first-fruits. And he also wrote that the truth of the thing is also that the fruit at the beginning of its planting until the fourth year is full of moisture that is very clingy, [which] injures the body and [so] is not good to eat; like the fish that have no scales, and the [other] foods prohibited by the Torah, which are bad also for the body.