Short answer. At their actual market value. Article 2133 provides that the actual market value of the fruits at the time they are applied to interest and principal is the measure of that application. So the harvest is credited at what it is really worth when applied, not at a figure fixed in advance or an inflated valuation.
What the law says
The actual market value of the fruits at the time of the application thereof to the interest and principal shall be the measure of such application
Civil Code, Article 2133 — Measure of Application. Read the full provision →
The yardstick is real market value
In an antichresis the creditor is given possession of the debtor's immovable and takes its fruits — a harvest, rents, produce — applying them to the debt. The obvious question is at what value those fruits count. Article 2133 answers it: The actual market value of the fruits at the time of the application thereof to the interest and principal shall be the measure of such application. The fruits are credited at what they would actually fetch in the market, not at a nominal figure the parties dreamed up or a rate that flatters one side. Real worth, objectively measured, is the standard the article imposes.
Why an objective measure protects both sides
Tying the credit to actual market value guards the debtor and the creditor alike. It stops a creditor from undervaluing a good harvest so that little is knocked off the debt while he keeps the surplus, and it stops a debtor from claiming a poor crop was worth far more than it was. Because the debt is reduced by whatever the fruits are truly worth, an honest accounting depends on an honest valuation, and the law fixes that valuation to the market rather than to either party's preference. Neither side gets to set the price of the fruits to suit itself.
Timing and order of application
The article fastens the valuation to a particular moment — the time of the application of the fruits to the debt — not the date of the contract or some other point. Market prices move, so when the fruits are credited can matter to how far they go. The application itself runs to interest and principal: the fruits are set against what is owed, reducing the debt by their market value as they are applied. Getting both the timing and the figure right is what makes the running balance of an antichresis accurate rather than a matter of assertion.
Keep the accounting straight
Because everything turns on value at the time of application, the useful discipline is a contemporaneous record: what fruits were gathered, when they were applied, and their market value at that point. That record is what lets either party check whether the debt has been reduced by the correct amount. Disputes in antichresis are, at bottom, accounting disputes — how much the fruits were worth and how they were credited — so the party who can show the market value at each application, rather than reconstruct it years later, is the one in a position to prove the balance.