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ADF See acid detergent fiber. 
ADG See average daily gain. 
ADIN Acid detergent insoluble nitrogen. Undigestible dietary protein or nitrogen. 
AP AM/PM. Abbreviations commonly used to describe alternate morning/evening monthly types of testing plans. Milk weights and samples are collected at the morning milking for 1 month and at the evening milking the following month.
Abomasum The fourth stomach compartment (true stomach) of a ruminant. 
Abortion Premature expulsion of the fetus or unborn animal. 
Acetonemia (ketosis) A condition characterized by an elevated concentration of ketone bodies in body tissues and fluids. It is more common among high-producing cows in a negative energy balance.
Acid detergent fiber Fiber measurement extracted with acidic detergent in a technique employed to help appraise the quality of forages. Includes cellulose, lignin, ADIN, and acid-insoluble ash.
Ad libitum (ad lib.) At pleasure. Commonly used to express feed available on free-choice basis. 
Aerobic Requiring oxygen. For example, many microorganisms require oxygen for the oxidation of food materials. 
Afterbirth The placenta and allied membranes with which the fetus is connected with the mother. It is expelled from the uterus following parturition.
Agalactia Failure to secrete milk following parturition. 
Age and Month-of-Calving Factors Factors used to eliminate the environmental effects of different ages and months of the year at calving. These factors standardize lactation records for genetic evaluations.
Age at Last Calving Age determined by subtracting the cow's birth date from her most recent calving date. 
AI Daughter Female offspring of an artificially inseminated dam. 
Alley A walking area for cattle within a barn, as a loafing alley or feeding alley. 
Alopecia Baldness; loss or deficiency of hair, natural or abnormal. 
Ammonia A colorless pungent gas, NH3, composed of nitrogen and hydrogen; its compounds are used as fertilizers. 
Ammonium An ion (NH4+) derived from ammonia (NH3). 
Anaerobic treatment lagoon A structure to treat animal waste by predominantly anaerobic biological action using anaerobic or facultative organisms, in the absence of air, for the purpose of reducing organic matter in wastes.
Anaphylactic shock (anaphylaxis) A state of immediate hypersensitivity following sensitization to a foreign protein or drug.
Animal Improvement Programs Laboratory (AIPL) A U.S. Department of Agriculture research service computing laboratory that calculates genetic evaluations of cows and sires using data collected through the DHI system.
Animal Unit A unit of measurement of livestock, the equivalent of one mature cow weighing 1,000 pounds. The measure is used in making comparisons of feed consumption and grazing.
Anorexia Lack or loss of appetite. 
Anthelmintic A remedy for destruction or elimination of parasitic worms. 
Antibiotic A metabolic product of one microorganism or a chemical that in low concentrations is detrimental to activities of other microorganisms. Penicillin, tetracycline, and streptomycin are antibiotics. Not effective against viruses.
Antibody A protein (modified type of blood serum) developed or synthesized by lymphoid tissue of the body in response to an antigen. Each antigen elicits production of a specific antibody.
Antigen A high-molecular-weight substance (usually protein) which, when foreign to the bloodstream of an animal, stimulates formation of a specific antibody.
Artificial insemination Placement of fresh or frozen male semen into the female mechanically without normal sexual contact. 
Aseptic Preventing or free from contamination by microorganisms. 
Aseptic packaging Method of filling packages without allowing the entry of microorganisms. Commonly used for sterile milk products to provide improved keeping quality.
As fed Refers to feed as it is consumed by an animal, including moisture. 
Atrophy A defect or failure of nutrition or physiologic function manifested as a wasting away or reduction in the size of cell, tissue, organ, or body part.
Automatic take-off A device for sensing the end of milk flow in the milking machine, for shutting off the milking vacuum, and for retracting the milking unit from the cow's udder.
Average daily gain The average amount of daily liveweight increase as applied to farm animals.