As animals eat, the first step in the digestion process is to reduce the particle size of the feed consumed. Mammals do this with their teeth. Since chickens do not have teeth, however, they rely on the gizzard for digestion.

Feed is mixed with saliva as a chicken consumes it. Once swallowed, it travels down the esophagus to the crop, where it is stored, and minimal digestion occurs. In natural settings, a bird will quickly consume a large quantity of feed and then move to a safe location to digest it. 

From the crop, the feed moves into the proventriculus. Digestive enzymes and acids are secreted and begin to digest the feed. The acidic environment of the proventriculus is not enough to fully digest large particles of feed like grains or fibrous materials like grass. As a result, the undigested material moves into the ventriculus, also known as the gizzard. The muscular walls of the gizzard, with the help of grit, grind the undigested feed into smaller particles that can be digested. Free-range birds will often pick up and swallow small stones from the ground, but confined chickens should be offered grit to aid in the digestion process. This is particularly important if the birds are consuming whole or cracked grains or grass. The small stones or grit will remain in the bird’s gizzard until they are ground down to a size small enough to pass through the remainder of the digestive tract.  

There is often confusion around the use of grit, limestone, and oyster shells in poultry diets. Grit is comprised of small stones, often granite or quartz, and is used to help grind the undigested feed — it offers no nutritional benefit beyond that. Grit is commercially available in small, medium, and large sizes for starting, growing, and finishing birds, respectively. Young chicks should be started on grit by sprinkling it on their feed twice a week. Growing chickens and hens should be provided grit free choice in a feed pan or trough.  

As compared to grit, limestone is offered to poultry as a source of calcium. While it can provide some grinding action, limestone dissolves and is absorbed before the undigested feed is completely ground. Laying hens have increased calcium requirements due to egg production. It is often necessary to provide supplemental calcium in the diet, even if a well-fortified feed is provided, to ensure adequate eggshell strength. This can be done by providing free-choice limestone to laying hens. 

Oyster shells are also a good source of supplemental calcium and can be offered free choice to laying hens as well. Like limestone, oyster shells provide little grinding assistance in the gizzard. It should also be noted that free-choice calcium supplements should not be provided to young, growing birds.

 While limestone and oyster shells offer the same benefit of calcium supplementation, they should not serve as a replacement for grit in the diet. Grit is particularly important to birds consuming scratch (or whole) grains or greens. The Co-op offers all three supplements in addition to a full line of complete poultry feeds and scratch grains. Call or visit your local Co-op for more information.