Breeding in Cattle Reproduction
The breeding program may use either artificial insemination (AI) or natural service. AI has been commercially available for >60 yr; it is widely used in dairy cattle but is used much less in beef cattle because of handling and labor costs. AI offers a selection of bulls with known genetic potential, such as measured by estimated breeding values (EBVs) for traits such as ease of calving or growth rates. When nutrition and heat detection are properly managed, satisfactory results are obtained. Failure to detect estrus is the major reason for unsuccessful AI. When cows are properly inseminated with good-quality semen at the proper time, 50%–60% may conceive on first service, the same percentage on second service.
Embryo transfer (see Embryo Transfer in Farm Animals) is frequently used to increase the number of progeny from the most valuable beef and dairy cows. Sexing of semen has been adapted to commercial field usage and is increasingly used. Sexing of embryos is becoming more readily available and practical for field usage, and some cloning techniques are emerging in availability.
Heifers should be bred according to size and age at puberty; at first breeding they should be 65%–70% of their projected mature body weight, with dairy heifers achieving >125 cm withers height at this stage. Selection of bulls to be used for natural service should be based on likely size of the calf at birth; the bull’s own birth weight (not his adult weight) is a useful guide, but genetic proofs such as EBVs are vital.
Heat synchronization of heifers and cows is possible, but such programs depend on adequate management and cooperation. Also, sufficient skilled labor to breed and assist during calving is essential.
In cattle, AI is used primarily for genetic improvement of livestock and to facilitate high health replacement strategies. The worldwide adoption of AI for genetic improvement in dairy cattle was made possible by development of a progeny test system and subsequent use of milk production records as an objective measure of performance on which to select superior bulls, techniques 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. The freezability of semen varies among bulls. 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.
(Also see Breeding Soundness Examination of the Male.) Semen is collected using an artificial vagina 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 spermatozoa 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 spermatozoa 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 spermatozoa can withstand a wide range of rates. In practice, extended semen is frozen in liquid nitrogen vapor 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 evaporates.
Because spermatozoa 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.
The rectovaginal method is used almost exclusively. After thoroughly cleaning the external genitalia with disposable toweling, one gloved hand is introduced into the rectum and grasps the cervix. The insemination pipette is introduced through the vulva and vagina 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 sperm 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 spermatozoa 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.