In agricultural parlance, a field is a portion of land belonging to a farm. In huntsman's language, it means all the riders.
In heraldry, it means the entire surface of the shield.
In military language, it means a battle; the place where a battle is fought, or is about to be fought; a campaign.
In sportsmen's language it means all the horses of any one race.
Against the field. In horse-racing, to bet against the field means to back a particular horse against all the rest entered for the race.
In the field. A competitor for a prize. A term in horse-races, as, so-and-so was in the field. Also in war, as, the French were in the field already.
Master of the field. In military parlance, means the conqueror in a battle. To keep back the field, is to keep back the riders.
To take the field. To move the army preparatory to battle. To win the field. To win the battle.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894