Sheep - AgRIS

Oral administration of L-thyroxine to Suffolk sheep brought about an increase in wool
production per unit area of skin due to an increase in fiber length growth rate and also caused
an increase in milk secretion during the second to eighth week of lactation in Black Leicester X
Scotch Black face ewes.
The main glucocorticoid secreted by the adrenal cortex of the sheep is cortisol and the
plasma level of cortisol in the sheep is exceptionally low compared to other species.
Administration of cortisol or ACTH to normal sheep suppresses wool growth and complete
inhibition can be produced even though the plasma cortisol level remains below that of other
species for which the plasma levels are known. Raised levels of plasma cortisol cause "breaks
and "tenderness" of fleece. Stress conditions such as injury, disease, starvation and extremes of
environmental temperature stimulates adrenal cortical secretion and a break in the fleece
following such conditions is probably medicated via the adrenal cortex.
Growth hormone having direct action, does not appear to have a permissive effect on wool
growth in that it does not permit growth in hypophysectomized sheep, more over it does
supplement the response to thyroid hormone in the sheep. Other pituitary hormones such as
prolactin, gonadotrophic hormones or oxytocin do not appear to have any significant
permissive or regulatory effect on wool.
e) Physiological state
It has significant effect on wool growth. In general entire males produce more wool than
castrated males which produce more than females. There is no real evidence, however, to
suggest that these differences are determined by anything other than differences in size and
nutritional state. The reproductive cycle in females can reduce wool growth significantly.
During late pregnancy, depression of wool growth rate in the range of 20-40 percent has been
found and even in breeds which exhibit a marked seasonal depression, pregnancy can depress
the rate still further. In lactating ewes, wool growth is also reduced by upto 30 percent or more,
despite the accepted increase in voluntary intake associated with lactation.
The average reduction in clean fleece weight between handicapped animals (twins and
progeny of young ewes) and unhandicapped (singles and the progeny of adults) animals born in
the same year was 150 g. The period most sensitive to nutrition deficit is from 30 days before to
35 days after birth (Fraser, 1954).
f) Age
Rate of wool production in most breeds increases to a maximum between four and five
years and then declines, often rapidly.
g) Nutrition
The most important factor affecting adult wool growth however, is the current level of
nutrition (Fraser, 1954; Kapoor et al., 1972) . Increase in energy intake, except at very low
levels of protein content in the diet, can have an immediate effect on wool growth detectable
within a few days. It is generally accepted that wool growth rate is dependent on energy rather
than protein intake. Wool protein contains a high proportion of the high sulphur amino acid,
cystine, and it has been shown that variation in the availability of the sulphur amino acids to the
follicle can affect both fiber growth rate and fiber composition.
It is not possible to estimate the actual amount of energy which produces wool. The
amount of metabolizable energy which is drawn upon for wool production is likely to exceed
materialy the combus tible energy for wool fiber and of the secretions which accompany it,
which is 3% of the metabolizable energy derived from rations (Marston, 1955). Deficiency of
minerals such as calcium and vitamin D are associated with depressed wool growth. These act
indirectly possibly by affecting health and appetite.
Carbohydrate supplementation to sheep increases body weight and fleece weight (Fraser,
1954). The addition of carbohydrate to diet converts a negative nitrogen balance into a positive
one (Krishnan, 1939). Carbohydrate is necessary for mitosis to take place and there is
importance of glycogen in the follicle for wool growth.
12.2.2 Genetic
There is variation in wool production in various breeds. In five wool breeds, winter growth
rate has been found to be around 85% of that in summer, whereas in British hill breeds the
winter rate may be as low as 30% of that in summer. The wool quality of Deccani and their
467