Abstract: An improved adjustable hog gate for use in forming pens is provided with a stationary section and an adjustable section which may be extended from, or retracted onto, the stationary section. The stationary section is bounded by an end plate which receives the lateral bars of the adjustable member while the adjustable section has an end plate which receives the lateral bars of the stationary member. The improved gate may be used in a confinement facility pen system which allows the central alley of the confinement facility which is defined between opposing series of pens to be adjustable in width. The alley may be expanded in width by urging each of the improved hog gates into a retracted position while animals are moved along the alley and then manually extending each of the invention gates to narrow the alley and provide a larger effective pen area for containment of hogs. An alternative embodiment provides a modification device for converting a fixed length hog gate to an adjustable hog gate.