The seed is the Gospel itself, which is preached openly to all. But it is effectual only in the sheep of God. Hence why Augustine points out that the gift of perseverance is reserved only for the elect, while others necessarily must fall away:
“But of such as these [the Elect] none perishes, because of all that the Father has given Him, He will lose none. John 6:39 Whoever, therefore, is of these does not perish at all; nor was any who perishes ever of these. For which reason it is said, They went out from among us, but they were not of us; for if they had been of us, they would certainly have continued with us. 1 John 2:19”. (Augustine, Treatise on the Predestination of the Saints)
“I assert, therefore, that the perseverance by which we persevere in Christ even to the end is the gift of God; and I call that the end by which is finished that life wherein alone there is peril of falling.” (Augustine, On the Perseverance of the Saints)
Here is the verse Augustine is referencing in its full:
1Jn 2:19 They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us.
Any who belonged to Christ must necessarily stay, and those who leave are never described as having once been of us, but of leaving us at some point because they were "not of us" to begin with.
I see no problem with Augustine's explanation but I would word yours differently like this:
The seed is the Gospel itself, which is preached openly to all. Those in whom it is effectual are Messiah's sheep. Those that persevere are the elect, while others fall away:
Exactly my point. Everyone has a chance.