This is from "The Puzzle of Agricultural Exceptionalism in International Trade Policy", by Michael Trebilcock and Kristen Pue:
... Hence, in explaining the resilience of the agricultural sector to efforts to liberalize trade in agricultural products we are left with a rather untidy amalgam of some elements of normative claims for agricultural exceptionalism along with positive political economy explanations for the disproportionate influence of agricultural producers in the political process.
Recognizing this resilience counsels patience and incrementalism in any attempt to unwind the extreme degrees of agricultural protectionism, especially in developed countries, and argues for a central role for transition cost mitigation strategies in moving to a more liberal international trading environment. In future multilateral or regional trade negotiations, we favour locking in substantial tariff reductions, implemented over an extended implementation period (e.g. 10 years, although much longer transition periods may be warranted for developing countries with large vulnerable, traditional, subsistence agricultural sectors where household incomes fall significantly below some national norm, e.g. the average manufacturing wage);
I've sometimes wondered whether long, slow liberalization periods are the answer to the current trade negotiation impasse at the WTO. What if WTO Members agreed to eliminate all tariffs (including anti-dumping tariffs) over a 50 year period? That's so far off that any negative political impact would be felt by a future political regime.