That's quite a stretch, isn't it?
After all, didn't Madison say the power was to be used "as a negative and preventive provision against injustice among the States themselves"? Where's the injustice by the state? Seems to me the injustice is being caused by the producer of unhealthy animals.
Didn't Madison also say that the injustice was the abuse of the power by the importing States in taxing the non-importing? He said nothing about unhealthy animals. There are many many references by Madison and other Founding Fathers that the abuses in question were due to state imposed taxes, tariffs, imposts, duties, etc.
Now, I agree that Congress may prohibit the interstate shipment of tainted or unhealthy beef. They did so in 1884, which was later upheld in Hoke v. United States, 227 U.S. 308 (1913). But that had nothing to do with any perceived injustice between producers. Justice McKenna delivered the opinion of the court:
"Our dual form of government has its perplexities, state and nation having different spheres of jurisdiction, as we have said; but it must be kept in mind that we are one people; and the powers reserved to the states and those conferred on the nation are adapted to be exercised, whether independently or concurrently, to promote the general welfare, material and moral. This is the effect of the decisions; and surely, if the facility of interstate transportation can be taken away from the demoralization of lotteries, the debasement of obscene literature, the contagion of diseased cattle or persons, the impurity of food and drugs, the like facility can be taken away from the systematic enticement to and the enslavement in prostitution and debauchery of women, and, more insistently, of girls."
Sounds to me as though Justice McKenna is talking about the positive purposes of the General Government.
In addition, in your favorite case, Hammer v Dagenhart, No. 704. Argued April 15, 16, 1918 June 3, 1918, Justice Day said in the opinion of the court:
"Articles heretofore barred and dealt with by this court have been such as could fairly be said to be "outlaws of commerce"; consequently all persons have been forbidden to ship them; the article itself is barred from commerce."
He did say that was an injustice, and that these injustices were the reason they considered giving the federal government the power to RCATSS. They also imagined that other "contrivances" might be implemented that would also constitute an injustice and should be dealt with by the same means. If Madison didn't say anything about unhealthy animals, which of the Founders did, that you enter this example into evidence?
And what direct bearing does this opinion have on the intent of the Founders when they wrote and ratified the Constitution? On what basis do you assert that this opinion rendered in 1913 by others, be considered their opinion in 1787? You have Justice McKenna describing the use of the power to RCATSS for the "positive purposes of the General Government". Now, when you can find one of the Founders describing this same kind of use you will have evidence to support your reading of Madison's letter. Until you do, you don't have it.