This material addresses aspects of the following syllabus outcome:
H2.2 describes the inputs, processes and interactions of animal production systems.
The work presented in the following section contributes towards achieving the following syllabus content areas:
Students learn about:
Animal reproduction and genetics
Extract from Stage 6 Agriculture Syllabus NSW Board of Studies Amended 2013
The following factors may affect the fertility of farm animals:
Genetics/heredity affects the fertility of farm animals in a variety of ways. Some animals may be genetically infertile.
sows can give birth to up to 20 piglets in one litter while cattle usually produce a single calf from each pregnancy
|Genetic differences do exist:
Genetic mutations can occur which can produce infertility. They can also give rise to abnormal development in embryos so much so that the young fail to develop properly and cause foetal death or atrophy. These are called lethal factors. Poultry seem to be more affected with lethal factors than other animals.
A peculiar form of sterility occurs in heifers born twin to a bull. In about 90% of such cases, the foetal circulations fuse. The male then remains normal but the female becomes a freemartin. To some extent, the female becomes masculinised by androgen produced by its twin.
Generally the heavier the animal the more fertile, but overfat animals may have difficulty mating, conceiving, and birthing. Puberty is determined by body weight not by age of the animal.
Poor nutrition may cause irregular cycles in females, reduced ovulation, weak offspring, pregnancy toxaemia or reduced twinning.
In males poor nutrition may reduce sperm quantity and quality.
The amount of food given to ewes immediately before they are joined is also of considerable importance. Experiments have shown that, if at that stage, a ewe is given a generous plane of nutrition she is likely to shed more ova than normal. This results in a higher lambing percentage by increasing the number of twin births. This procedure is known as flushing. There is evidence to suggest that the numbers of sperm produced by rams may be increased by feeding a high energy and high protein diet for about six weeks before the mating season.
The amount of food given to the pregnant mother also influences birth weight of offspring and heavier offspring have a better chance of surviving. Conversely, the undersized new born animal has less chance of surviving and frequently dies in the first few days following birth. Such an animal is less able to maintain its body temperature and may die quickly if born in cold weather.
If over-fed, the pregnant animal may suffer difficulties in birth which could also lead to death of either the offspring or mother or both.
In order to avoid pregnancy toxaemia this ewe was steamed up in the latter stages of pregnancy.
|Under nutrition in late pregnancy, particularly in ewes
carrying twins, may cause pregnancy toxaemia. This has given rise to the
practice of steaming up.
Steaming up is feeding technique where females are put on a rising plane of nutrition in the latter stage of pregnancy. It increases birth weight and milk production.
Whilst deficiencies in protein may cause infertility, it has also been shown that copper deficiencies in some eastern states of Australia causes infertility in cows while a deficiency in vitamin A prevents normal sperm production in bulls and rams in some areas.
The effect of age is closely linked with nutrition, as puberty is determined by body weight not by the age of the animal. The physical size of animals may affect their ability to mate, carry a foetus or give normal birth to offspring.
After puberty, fertility generally increases for some time then decreases when the animal gets too old. In contrast, goats have a greater chance of twin kids as they age.
Some species are polyoestrous (pigs and cattle), this means that they cycle throughout the year. Other species are seasonally polyoestrous meaning that they breed only during particular months of the year (horses, sheep, poultry and goats). The decreasing daylight hours stimulate ewes and does to cycle while the increasing daylight hours stimulate hens and mares to cycle.
|Some deer and primitive sheep breeds have only one
oestrous cycle which occurs at a particular time of year and so are
Dogs have only one cycle which is unrelated to the season and so are monoestrous.
The period of the year at which animals breed has evolved by natural selection to ensure the young are born at a time giving them the best chance of survivial. In sheep, the mechanism by which the breeding season is brought about is triggered by hours of decreasing daylight. This stimulates the anterior pituitary gland to produce FSH. Experiments have shown that, by the use of electric light, sheep in the Northern Hemisphere can be made to breed at the same time of the year as sheep in the Southern Hemisphere. When ewes are taken across the equator from north to south, they change their breeding season to correspond with the hours of daylight.
The daylength is also a factor with the production of sperm in the ram but does not seem to be in other animals.
The introduction of Bos indicus cattle into the tropical areas of Australia is an example of using climatically adapted animals in order to maintain fertility and hence production
|High body temperatures produced in rams by high summer
temperatures is a cause of poor quality semen. This also affects semen
formation in bulls.
High temperatures can also affect mating with reduced sexual activity. Conception rate is also reduced. This affects the number of offspring born.
In males, the ability of the scrotum to reduce the temperature of the testes sufficiently to produce sperm of sufficient number and viability is severely restricted.
It is important, therefore, to have both climatically adapted animals and to provide conditions conducive to proper spermatogenesis to ensure maximum fertility.
Disease as a factor that can affect the fertility of farm animals can be related to nutrition, climate, management and genetics. Disease may stop sperm or egg production, may cause abortion, may reduce the health of the foetus so reducing the chance of its survival.
Infective conditions include brucellosis, trichomoniasis, leptospirosis and vibriosis. These diseases have dramatically reduced in recent years due to diligent eradication campaigns.
Persistance of the corpus luteum, usually as a result of infection can be a problem particularly in cows. This prevents the production of FSH and consequently normally cycling does not occur.
|Some varieties of subterranean clover contain high levels of oestrogen. When sheep eat pasture containing high percentages of these varieties they become infertile. This condition is commonly known as clover disease. Subterranean clover varieties that contain high levels of oestrogen include Dwalganup, Yarloop and Dinninup.|
The farm manager has the ability to control or at least to maniupulate the factors that have been discussed above. Examples of how farmers control or manipulate these factors include:
|Selecting sheltered paddocks for ewes at
lambing time can assist good lamb survival rates
Control of parasites assists in maintaining breeding stock in good health to ensure maximum fertility rates.