פירוש על קידושין 63:3

They may redeem the second tithe for each other: If his father has second tithe, the son may redeem it with his [money] for what his father needs, without [the extra fee of] a fifth. And we do not say, behold he is like [the father] himself: As surely he loves him and his sustenance is upon him, so he is not a different [person] concerning him; and behold it is like someone who redeems it himself.

And they may feed each other poor man’s tithe: If the son has poor man's tithe to distribute to the poor, he may feed it to his father if his father is poor. And likewise, all of them. And if he did not have have this tithe in his hands, he must give him nourishment from his [funds].

Every third and sixth year of the sabbatical cycle a Jew must give 10 per cent of his produce to the poor. But a son can give it to his poor father. The problem is that one should not be filling debts with the poor person’s tithe. So this would imply that feeding one’s father is not the responsibility of the son. This therefore is a difficulty on Rav Judah from above.
The resolution is that the son is giving him extra food from the poor tithe, not the basic amount he is obligated to give him by law.