Unit 3

Life Cycle Considerations -- Puberty, Estrous Cycle & Breeding

In Unit 2, we discussed a number of different livestock management systems.  Among the characteristics of each management system examined was the life cycle considerations of the animals being "managed".  The reproductive cycle is a very important consideration in several different management systems.  Of the following systems, note which ones rely on reproduction to meet their intended use and come to class prepared to defend your evaluation.

    Puberty is the time in an animal's life cycle when the animal reaches a developmental state in which it is capable of reproducing.  By defination, it is the time in adolescence when male and female gonads are capable of releasing gamates. This happens several weeks before sexual maturity; the first gamates are usually incapable of fertilization.  In a production system that relies on reproduction to be profitable, the sooner an animal reaches puberty, the sooner the manager will be able to realize a profit from the animal.

    A number of factors influence the age at which an animal reaches puberty.  Among the factors are:

• Animal species - smaller species usually reach puberty before larger species
• Breed - some breeds within a species reach puberty earlier than other breeds
• in cattle, Hereford and Angus reach puberty before Brahma and Zebu
• in swine, Landrace reach puberty earlier than other breeds
• Climate - animal generally have an earlier onset of puberty in warmer climates, however excessive heat may actually delay the onset of puberty
• Season - many breeds and species of animals are seasonal breeders
• Long-day breeders begin to cycle and breed during  increasing day length
• Horses, chickens, swine are examples of long-day breeders.  While this trait has been bred out of many of our domesticated swine and chickens, a decrease in fertility (fall infertility syndrome, or fall abortion syndrome in swine) is thought to be due, at least in part, to the effect of decreasing day length.
• Short-day breeders begin to cycle and breed and are most fertile during decreasing day length
• Sheep, goats, deer, and elk are examples of species who are normally fertile and actively breeding in the fall of the year during decreasing day length.  Exceptions to this in the Ovine species are the  Dorset and Ramboullet breeds of sheep, who have been selected to remain fertile and capable of breeding at other times of the year.
• Nutrition - an adequate, well-balanced diet is necessary for animals to reach puberty at an optimal time.  Severely malnurished animals will have a delayed onset of puberty.
• Sex - the female of the species or breed usually reaches puberty earlier than male animal.
• Management system - the type of housing, pen arrangement, lighting, and the presence or absence of mature animals will affect the onset of puberty.
• the sight, smell and sound of the opposite sex enhances the onset of puberty in swine
• Stress - recent studies indicate that low levels of stress results in early onset of puberty, excess stress, however, may delay the onset of puberty

    Once animals reach puberty, we tend to concentrate our attention on the female, since she is producing the desired product.  It is not necessary to keep as many males as females for breeding purposes, and when artificial insemination (AI) is used, there may be no intact male animals in a livestock production system.  Through use of AI, new genetics can be introduced into the herd or flock without running the risk of inroducing disease agents that may be carried by an apparently healthy animal.  In addition, male animals tend to be aggressive and unpredictable, often fighting among themselves.  However, until or unless cloning of animals from somatic cells becomes economically feasible and socially acceptable the male will continue to play an important role in reproduction.

The Role of the Male in Reproduction

A number of factors effect male reproductive performance.  Among the factors are:

• Conformation of the male animal.  Male animals that are used in natural breeding programs are required to find receptive females, mount them and breed them.  If the animal is of poor conformation, it may become lame or unwilling to shift it's weight to the rear legs at time of mounting and breeding.
• Health - the overall health of the animal can also affect it's reproductive performance.  A sick animal will also lack the stamina to search out and breed receptive females within the herd or flock.
• Environment - HEAT adversely affects the male's ability to breed in two different aspects.  The exertion of seeking out and breeding receptive females place additional heat stress on the male.  In addition, most male animals are unable to produce optimal numbers of viable spermatazoa at high environmental temperatures.  The testes of most male species are normally housed in the scrotum which is exterior to the body.  As a consequence, the temperature of optimal sperm production is below that of the normal body temperature of 100-102^oF.  When environmental temperatures reach or exceed the normal body temperature of the animal, production and survival of sperm within the testes is decreased.  The exception to this is the avian species, in which  testicles are located within the abdomen and the normal body temperature (103-107^oF) is higher.
• Semen quality - just because an animal appears to be in good health, there is no guarantee that the animal is producing high quality semen.  It is a good practice to check the semen quality of a stud animal prior to his use.
• Intended use/age of sire - the number of females that a male animal can successfully breed in a given period of time depends upon the intended use, age and experience of the sire.  Young, inexperienced males are not able to successfully breed as many females in a given time period as an older, more experienced male.
• Type of breeding system -  the largest number of males are required per given number of females when a pasture breeding system is used because the males have to exert a great deal of energy seeking out and identifying receptive females.  If the system manager identifies receptive females (females in heat) and brings them to the male for "hand mating", fewer males will be necessary to breed a given number of females.  When using artificial insemination, still more females can be bred with a single male animal, because each collection of semen can be split and used to inseminate several females.
• Length of breeding season - it will take more males to breed a given number of females if they all need to be bred within a 1 month period of time.  If a 2-3 month breeding window is used, fewer males will be needed.

Examples of numbers of females that can be successfully bred by a male animal follow:

• Young bull on pasture -15 females/45-90 day season
• Mature bull on pasture - 25-40 females/45-90 day season,  4-12/week if hand breeding

 

• Ram lamb (9 mo. old) - 5-10 ewes/breeding season
• Yearling ram -  25-30 ewes/breeding season
• Mature ram may be used up to 3X weekly

 

• Yearling boar - may be used to breed females 3X weekly
• Mature boar - may be used to breed females 1X day

    Once the male animal reaches puberty, production of spermatazoa is a more or less constant process.  There are, however, seasonal decreases in the day-length dependant species and breeds of animals.  This means that unless the male is overused, he is capable of breeding and impregnating a female at any given time.  When the female animal reaches puberty, she begins to establish an estrous cycle.  In contrast to the male, she will only be receptive and able to become pregnant during a short period of time at a critical point in the estrous cycle.  Among the livestock species, an estrous cycle occurs in cows, sheep, swine, horses, does, etc.    The avian species are exceptions to this and their reproduction will be discussed separately.
 

The Estrous Cycle -- A Female Thing

    The estrous cycle is divided into 4 phases - estrus, metestrus, diestrus and proestrus.  An additional period of anestrus (no estrus) occurs in some animals, and is located between diestrus or partrition and proestrus.  Because females can only become pregnant and produce offspring during a short interval within the estrous cycle, it is very important to understand the cycle and be able to identify animals at critical stages of the cycle.

Heat detection is an important livestock management tool -- missed heats (estrous cycles in which an animal remains "open" or is not bred) result in:

• Increased parturition intervals, decreased numbers of offspring produced per year or lifespan of the female
• Longer dry periods and consequently decreased milk production per year or lifespan of a diary cow
• Less use and reduced efficiency of facilities (milking parlors, farrowing barns, calving and lambing sheds, etc.)
• Reduced profits due to the costs of upkeep for animals that are not producing at their optimal efficiency

Detecting estrus is a challenge to all livestock production systems relying upon reproduction for their survival.  Some livestock systems keep intact male animals and intermingle them with the females, letting the males do the job of heat detection.  However, if hand breeding or artificial insemination is used, it is important for the herdsperson to be able to identify animals that are in estrus.  Also, it is important to know the approximate time of parturition, and in order to predict that, one must know when the female was bred.  Consequently, observation and recording of estrus and breeding activity are important management practices in a livestock operation.  There are a number of behavioral and physical signs that one can use to identify an animal in the estrus phase of the estrous cycle.

Currently, the best means of heat detection is observation of the animals.

• Observation should be done during the animals' normal activities
• Observation is usually done during morning hours, when the animals are normally more active.  (With cattle, observation must be done twice daily, or estrus may be missed due to the short 18 hour length of estrus)
• Recording of dates and times of estrus activity will allow the herdsperson to carefully observe the animal at the anticipated time of estrus in the next cycle.
• Recording of the time of certain events such as weaning, will allow the herdsperson to carefully observe groups of animals in which estrus would be expected within a few days.  For example, animals that are suckling offspring will often exhibit anestrus (no cyclic activity) until the offspring are weaned.  At that time the animal will rapidly enter proestrus and will exhibit signs of estrus within a few days.

The primary sign of estrus is the female animal standing to be mounted.  This is the most important and most reliable sign of estrus and receptivity for breeding to establish pregnancy.  Some animals, however will not exhibit this primary sign and the herdsperson will have to rely on secondary signs of estrus to determine when to breed the female.

• Behavioral signs of estrus
• Restlessmess and nervousness
• Isolation from the rest of the herd or flock
• Tail elevated, ears perked (cattle & horses)
• Increased interaction with other members of the herd or flock
• Mounting of other members of the herd or flock

• Physical signs of estrus
• Thick, clear, stringy mucus from vulva
• Reddened, swollen vulva
• Roughened, dirty haircoat from being mounted
• Reduced milk production (this is normally monitored in dairy cattle)

 

Not all, if any, signs may appear during a single estrus in an individual animal.  Herd managers will establish through experience, the combinations of signs that most reliably indicate estrus in their herd or flock and will use them to identify animals to be bred.  The best way of detecting estrus is through observation of the animals and their interactions.   A number of other devices, however, are available to aid the manager in this task.

• "Teasing" of the female animal, by bringing a male within close proximity (providing the sight, smell and sound of the male) will often stimulate her to exhibit behavioral signs of estrus.  This is helpful with animals that may be housed singly and don't have the opportunity to interact with others of their species on a daily basis.
• "Heat patches" placed on the rump of animals that will be broken and release a dye when the animal is mounted can be used to mark individuals who have stood to be mounted by herdsmates.
• Rams and bulls can be outfitted with chin-ball markers or harnesses that will leave a mark on the rump or back of animals that they have successfully mounted, giving the herdsperson an indication of which animals in the herd or flock have been bred by the stud.
• Estrus synchronization by use of hormone injections/implantations.   Hormone injections can be used to stimulate growth of follicles on the ovary, cause lysis of the CL and cause ovulation and release of the ovum from the follicle.  This allows the herd manager to synchronize the estrus of a group of animals and breed them to calve at the same time..

 
 

General Guidelines of Breeding by Species

 

Puberty Onset

Estrus Cycle

 

Species	Ave. Age	Age Range	Ave. Length	Range	Estrus (Heat) Duration
Cow	12 months	4-18 months	21 days	18-24 days	18 hours
Mare	12 months	10-24 months	21 days	19-26 days	6 days
Ewe	9 months	5-12 months	16.5 days	14-20 days	30 hours
Sow	7 months	4-9 months	21 days	18-24 days	2-3 days

 

The best time to breed in relation to the observed estrus varies, depending upon:

 1.  Survival time of the egg post-ovulation
 2.  Survival time of sperm within the female reproduction tract.  In many species, spermatazoa must be subjected to the secretions within the female reproductive tract for a certain amount of time before they ares capable of fertilizing the egg.  This is called capacitation.
 3.  When ovulation takes place.  Remember that not all species ovulate at estrus; the cow ovulates after estrus.

 
 

Species	Best Time to Breed	Time of Ovulation
Cow	mid-heat (9th hour)	10-12 hours post-heat
Mare	two times, two days apart	1-2 days before heat is ended
Ewe	anytime during heat	24-30 hours after heat begins
Sow	2nd day of heat	46 hours after heat begins

 
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