With the firstling of thy bullock: this is meant, either,
1. Of the male firstlings; which, they say, is forbidden here, because some did plough with the firstlings of their oxen, and shear the firstlings of their sheep, before they were offered. But this seems absurd and incredible, because they were to be offered on the eighth day, Exodus 22:30, when they were very unfit for such uses. Or rather,
2. The second firstlings, of which see on Deuteronomy 12:17.