The Dehesa is an Oak forest derived from the Mediterranean forest ecosystem containing at least 10 trees per hectare (10,000 sq meters). These Oaks are protected and pruned to produce acorns, which the famous black Iberian pigs graze on during the acorn season (Montanera) which lasts from September through to March. 

The weight of the pig is strictly controlled and upon entering the Montanera the pig must weigh between 92kg and 115kg. During the Montanera, the pigs gain at least 46kg feeding only on acorns and grazing the forest.  

Each year, the farmers face a difficult problem - how many pig should they allow into the Dehesa? Too many and there will not be enough acorns for the pigs to reach their desired weight. Too few and acorns go to waste. In an average year, each pig requires at least 2 hectares of Dehesa to gain the required weight. However, as the acorn crop can differ significantly every year, this is quite a challenge. The reward is that ham produced from Iberian pigs are highly sought after and sell at premium prices, especially if acorns have been used for fattening.