Cattle Breeding

echniques for freezing semen, and liquid nitrogen storage refrigerators. The development of objective systems such as EBVs to measure economic traits in beef cattle (eg, growth rate, carcass conformation and composition, efficiency of feed conversion) and thus the more accurate selection of sires, as well as control of the estrous cycle, is leading to an increase in use of AI in beef cattle. Processing of frozen semen is a highly specialized technique. Attention to detail at each step is important to maintain semen quality. However, semen of high motility and morphology quality generally freezes well. Best results are obtained when semen is processed in a properly equipped laboratory by experienced staff at an AI center. Collection and Handling of the Semen Sample:
(Also see Breeding Soundness Examination of the Male.) Semen is collected using an artificial va**na or by electroejaculation (electrical stimulation of the seminal vesicles and ampullae). As long as the sample is of high quality, freezability and fertility should be normal. These techniques should not be used if the bull is unable to naturally service a cow for reasons that could be genetic. Most AI in cattle today is performed with frozen semen. Frozen semen may be maintained for years; extenders permit more insemination doses to be processed from one collection of semen, maintain the fertility of the semen longer, protect the s***matozoa from sudden temperature or pH change, and prolong viability. Semen is usually extended with citrate-buffered egg yolk or heat-treated skim milk plus glycerol, sugars, enzymes, and antibiotics. Final extension is designed to package 0.25 mL or 0.5 mL of semen containing 20–30 million s***matozoa at time of freezing. Extenders are often divided into fraction A and fraction B. The initial extension of semen is done with fraction A at the same temperature, eg, 86°F (30°C). The extended semen is then cooled to 41°F (5°C) over 40–50 min, or more slowly. Holding the extended semen at this temperature for 3–4 hr enables the antibiotics in fraction A to complete their action before being inhibited by the cryoprotectant glycerol. Fraction B contains a cryoprotectant such as ethylene glycol or glycerol (eg, 14%) and is added at 5°C in equal quantity to the extended semen. Each AI center has its own standard extenders and processing procedures. Glycerol (11%–13%) may be used with milk-based diluents. Before freezing, semen should be stored for 4–18 hr at 5°C. For freezing, bull semen is usually packaged in appropriately identified plastic straws (0.25 or 0.5 mL). Optimal freezing rates are known for many cell types, and s***matozoa can withstand a wide range of rates. In practice, extended semen is frozen in liquid nitrogen v***r before being plunged into liquid nitrogen at –320°F (–196°C). Storage in liquid nitrogen tanks is safe for ≥20 yr, and semen is transported in such tanks. The level of liquid nitrogen in tanks must be monitored to avoid semen losses, which is seen when the tanks become defective or when liquid nitrogen gradually ev***rates. Because s***matozoa do not survive for long after thawing, the semen should be used immediately. Thawing is best done as quickly as possible without damaging the semen by overheating. In practice, straws may be thawed in warm water (95°–98°F [35°–36.5°C]) for ≥30 sec and immediately placed in the cow’s reproductive tract. Recommendations by the AI center that processed the semen should be followed. Insemination Technique:
The rectova**nal method is used almost exclusively. After thoroughly cleaning the external genitalia with disposable toweling, one gloved hand is introduced into the re**um and grasps the cervix. The insemination pipette is introduced through the v***a and va**na to the external cervical os. By manipulating the cervix, along with light cranial pressure on the pipette, the pipette is advanced through the annular rings of the cervix to the junction of the internal cervical os and the body of the uterus. The semen should be expelled slowly (5 sec) to avoid s***m loss. If insemination records and consistency of the cervical mucus suggest possible pregnancy, the pipette should be advanced less than one-half of the way through the cervix, and the semen expelled. The optimal time to inseminate is between the last half of standing estrus and 6 hr thereafter, which is described as the "am/pm rule," in that cows observed beginning standing heat in the morning should be inseminated that afternoon, etc. If fertility problems arise when AI is being used, the semen should be investigated, although many factors other than semen are involved in attaining high fertility. Motility after thawing is an important criterion. An adequate number of motile s***matozoa at the time of insemination is critical. Morphologic examination also helps assess the role of semen in infertility cases. Comparisons within herds of diagnosable pregnancies resulting from the suspect semen and from semen from other bulls may be useful. Estrus detection continues to be the most important factor that influences AI efficiency. This factor should be investigated first, and inseminator proficiency second. The latter includes an evaluation of thawing temperature, time of thawing in relation to actual insemination, temperature changes from thawing to insemination, site and speed of semen deposition, and sanitary procedures. If semen is purchased from a reputable supplier, it is unusual for the cause of the infertility to be poor-quality semen, although transport and storage factors should be considered.