Bull Name	BW Acc	WW Acc	YW Acc	YH Acc	SC Acc	CEM Acc	Milk Acc	Marb Acc	Fat Acc
VT Pride	+4.4 .42	+40 .30	+60 .21	+.5 .16	–.62 .18	+8 .14	+20 .26	+.15 .14	+.026 .14
Wyatt’s Boy	+1.3 .98	+36 .97	+76 .96	+0 .96	–.55 .96	+7 .93	+16 .96	+.08 .87	–.023 .85

Note: These are hypothetical examples. You must consult each individual breed to define estimated progeny difference information.

BW (birth weight; expressed in pounds) is a predictor of a sire’s ability to transmit birth weight to his progeny compared to that of other sires.

WW (weaning weight; expressed in pounds) is a predictor of a sire’s ability to transmit weaning growth to his progeny compared to that of other sires.

YW (yearling weight; expressed in pounds) is a predictor of a sire’s ability to transmit yearling growth to his progeny compared to that of other sires.

YH (yearling height; expressed in inches) is a predictor of a sire’s ability to transmit yearling height compared to that of other sires.

SC (scrotal circumference; expressed in centimeters) is a predictor of the difference in transmitting ability for scrotal size compared to that of other sires.

CEM (calving ease maternal; expressed as a difference in percentage of unassisted births) predicts the average ease with which a sire’s daughters will calve as first-calf heifers when compared to daughters of other sires. A higher value indicates greater calving ease in first-calf daughters.

Milk (maternal milk) is a predictor of a sire’s genetic merit for milk and mothering ability as expressed in his daughters compared to daughters of other sires. It is the part of a calf’s weaning weight attributed to milk and mothering ability.

Marb (marbling) is expressed as a fraction of the difference in USDA marbling score of a sire’s progeny compared to progeny of other sires.

Fat (fat thickness; expressed in inches) is a predictor of the difference in external fat thickness at the twelfth rib (as measured between the twelfth and thirteenth ribs) of a sire’s progeny compared to progeny of other sires.

This information is statistically evaluated, and each bull receives a number for each characteristic. The number will have a + or – associated with it. The number then can be used to compare two different bulls. For example, if the VT Pride bull has a +4.4 for birth weight and Wyatt’s Boy has a +1.3, you could expect calves born to VT Pride to weight 3 lb more than calves born to Wyatt’s Boy (+4.3 – +1.3 = +3). Of course these numbers are averages, and bulls must be randomly mated to cows. The units for each characteristic vary, so producers and veterinary professionals should understand the units associated with each EPD.

The bull’s female offspring can contribute data on milk production and mothering ability. EPDs can be used only within the breed. You cannot compare a Hereford bull to an Angus bull. The information provided by EPDs can help a producer decide which of the bulls would be the best for his or her production system.

When looking at EPDs, the individual producer’s production system, amount of labor, and feed availability will contribute greatly to the producer’s choices.

Each EPD has an accuracy number associated with its value. The accuracy number (Acc) gives the producer an indication of the actual accuracy of the EPD. An accuracy close to 1.0 indicates higher reliability. Accuracy is affected by the number of progeny and ancestral records included in the analysis. The more progeny available for data collection, the greater the accuracy of the EPD (Table 3-1).

Estimated Breeding Values

Estimated breeding values (EBVs) are often used in the swine and cattle industry to determine the value of a breeding between two animals before the breeding takes place. The concept is the application of genetic principles to performance records. Producers often assess the performance of the animal being evaluated for breeding. Essentially the data used are similar to that collected in EPDs, but the degree of heritability of these traits is applied. For example, certain traits have a higher potential to be inherited than others. Thus, with knowledge of the potential heritability, the overall value of the animals being bred can be estimated by looking at which superior traits the animals have and how likely those traits are to be inherited.

Breeders can use EBVs to predict the outcome of a specific mating. By simply averaging the EBVs of the parents the genetic merit (value) of the progeny can be determined. Think of the following situation. A boar has an EBV of –6.0 days and a sow has an EBV of –2.0 days for reaching 90 kg. When these two animals are mated, on average the offspring will take 4 days less (–6 + –2 = –8; –8 ÷ 2 = –4) to reach 90 kg than the progeny of other boars and sows with the breed average.

You must consider all the data and understand the value and units as well as the accuracy of these numbers, just as for EPDs, in order for them to be valuable.

Pedigree

Pedigree is also used in determining which male or female should be bred. When decisions are made based on pedigree, producers often will evaluate the performance of the animal as well. For example, if a producer were looking to breed a horse that would produce a colt adept at racing, the producer might want to evaluate the performance record of the animal in the pedigree of that stud. Finding that the stud had Seattle Slew and Dash for Cash in his history could indicate that the foal produced from the mating would have speed and be good at racing. The concept can be considered as follows: Would you expect Michael Jordan’s son or my son to be better at basketball? My son is short, and neither of his parents have any basketball skills. The concept does not always work but is one of the bases used to select animals for breeding. This approach is often used in the horse industry. Pedigrees can also be used to select a type of breeding. These systems include inbreeding, linebreeding, crossbreeding, purebreeding, outcrossing, and grading up.

Breeding Systems

Reproductive Physiology

Veterinary technicians should be familiar with the estrous cycle of all species. All livestock species follow a similar estrous behavior, with slight differences in the hours between each landmark within the estrous cycle. Estrous cycle data are discussed in each species section.

The estrous cycle is broke into four phases: proestrus, estrus, metaestrus, and diestrus. Anestrus in livestock can be seen in seasonally polyestrous species and only in polyestrous species during pregnancy (Table 3-2).

TABLE 3-2

Female Reproductive Hormones

Hormone	Source	Female Target Tissue	Female Primary Action
Gonadotropin-releasing hormone (GnRH)	Hypothalamus	Anterior pituitary	Release FSH and LH from anterior pituitary gland
Luteinizing hormone (LH)	Anterior pituitary	Ovary (cells of theca interna and luteal cells)	Stimulates ovulation, corpus lutea formation, and progesterone secretion
Follicle-stimulating hormone (FSH)	Anterior pituitary	Ovary (granulosa cells)	Follicular development and estradiol synthesis
Estradiol (E2)	Granulosa cells of follicle	Hypothalamus, entire reproductive tract, mammary gland	Sexual behavior, GnRH, elevated secretory activity of entire tract, enhanced uterine motility
Progesterone (P4)	Corpus luteum	Uterine endometrium, mammary gland, myometrium, hypothalamus	Endometrial secretion, inhibits GnRH release, inhibits reproductive behavior, promotes maintenance of pregnancy
Prostaglandin F_2α (PGF_2α)	Uterine endometrium	Corpus luteum, uterine myometrium, ovulatory follicles	Luteolysis, promotes uterine tone and contraction, causing ovulation

Technician Note

A thorough understanding of hormones, their actions in the body, and their role in the estrous cycle will help technicians to better understand hormone therapies used in veterinary medicine and improve client education.

Estrous Cycle and Hormone Involvement

Proestrus

Proestrus begins with a decline in progesterone level, which results because the animal has just undergone a surge of prostaglandin F_2α (PGF_2α) in diestrus. The prostaglandin causes lysis of the corpus luteum, which results in the decrease in progesterone. Progesterone creates a block on gonadotropin-releasing hormone (GnRH), which then blocks follicle-stimulating hormone (FSH) and luteinizing hormone (LH). When the progesterone level decreases at the beginning of proestrus, the block is removed. By unblocking GnRH, FSH production begins. FSH causes the development of follicles on the ovary and with it the development of follicular fluid, which results in increasing estrogen.

Estrus

Once estrogen reaches a specific blood level in the body, it triggers the release of LH. Luteinizing hormone triggers ovulation. The follicle ruptures, and the egg is released from the follicle. The egg travels down the tube to the infundibulum where, if the animal has been bred, fertilization of the egg will take place.

Metaestrus

Because the follicle has been ruptured, the estrogen levels in the body will decrease because the follicular fluid is gone. The follicle becomes the corpus luteum, and production of progesterone begins. Metaestrus is the transition from estrogen being the predominant hormone to progesterone being the predominant hormone.

Diestrus

Progesterone continues to be produced until the end of gestation if the animal is pregnant. If pregnancy does not take place, the body triggers a prostaglandin surge of PGF_2α. The prostaglandin causes lysis of the corpus luteum, and progesterone begins to decline, which results in the transition from diestrus to proestrus (Fig. 3-1).

Breeding Soundness Examination

Owners must continually evaluate the reproductive performance of breeding males and females in the herd. The primary goal of a breeding soundness examination is to evaluate the current state of fertility of an individual, realizing that many factors can affect fertility. Animals with fertility problems often are candidates for culling because they would result in an economic loss. The breeding soundness examination is only one piece of information used in formulating a farm’s breeding management program.

The breeding soundness examination usually is performed just before the breeding season. In addition to a general health evaluation, the examination emphasizes the physical ability to breed females (especially eyes, legs, and feet). Examination of the external genitalia, sperm analysis, and a rectal examination of the internal accessory sex glands may be done.

Examination of the external genitalia must be done carefully to prevent injury to the examiner. Most males tend to resent handling of the genital areas. All males should be approached with caution and adequately restrained. The examiner is safest when positioned next to the animal’s chest. Standing next to the hindquarters should be avoided if possible; even heavily tranquilized animals can kick with the hindlimbs. In order to evaluate the penis, it must be extended from the prepuce. Some males allow manual extension by inserting a gloved hand into the prepuce and gently grasping the penis; however, this is uncommon. Most males must be tranquilized to relax the retractor penis muscle and allow the penis to extend. Rarely, after tranquilization, the penis remains extended for a prolonged period; this is more likely to occur when phenothiazine-based tranquilizers are used for tranquilization. Regardless of the drugs used, veterinary attention should be sought if extension persists for longer than 2 hours. Prolonged extension interferes with venous and lymphatic drainage of the prepuce and penis, resulting in rapid development of severe edema. Permanent damage to the penis (paralysis, paraphimosis, external trauma, priapism) may result if treatment is not instituted promptly (Box 3-1).

BOX 3-1 Potential Damage to the Penis

Penile paralysis: Inability to retract the penis into the prepuce due to nerve or muscle (retractor penis muscle) disease

Phimosis: Inability to extend the penis from the prepuce, usually due to excessive swelling of the prepuce, which prevents the penis from exiting

Paraphimosis: Inability to retract the penis back into the prepuce, usually due to excessive swelling of the prepuce and/or penis

Priapism: Prolonged erection unrelated to sexual desire, usually due to failure of the blood to exit the erectile tissue of the glans penis; swelling of prepuce and penis develop within hours and result in paraphimosis in addition to priapism

Technician Note

Prolonged penile extension is more likely with the use of phenothiazine-based tranquilizers.

The following parameters are evaluated during a male breeding examination.

The prepuce is commonly referred to as the sheath in large animals. The ability to extend the penis, extension of the penis in a straight line, and skin lesions of the prepuce or glans penis should be evaluated (Fig. 3-2). The penis and prepuce are susceptible to a variety of benign and malignant tumors, parasitic lesions (habronemiasis), and other growths (Fig. 3-3). These lesions have no typical appearance; any abnormality is suspicious and best evaluated by a veterinarian. Visualization of the penis of bulls is usually done during the semen collection procedure. Rams are set up on their rump. The external preputial ring is pushed caudally and down toward the abdomen to reveal the glans penis, which is gently grasped with a gauze square and pulled into extension. Horses should be examined during penile cleaning.

FIGURE 3-2 Examination of the prepuce and penis is a component of the reproductive examination.

Scrotum and Testicles

The shape, size, and consistency of the scrotum and testicles should be evaluated.

Measurement of Scrotal Circumference

Many factors such as age, breed, and time of year can affect the scrotal circumference. Scrotal measuring tapes can be commercially obtained. Measurement is made by pulling the testicles fully into the scrotal sacs and measuring snugly around the largest circumference point (Figs. 3-4 and 3-5 and Table 3-3).

TABLE 3-3

Expected Scrotal Circumferences

Bull Age (months)	Size (cm)
12–14	30–34
15–20	31–36
21–30	32–38
≥31	34–39
Ram by Weight (kg)	Size (cm)
<45	23–27
45–70	27–33
70–90	30–36
90–115	31–37
115–135	33–38
135–160	36–40