Proceso licitación de ERP

gradual, taking

2

years. In contrast, the effect of reversed photoperiodic seasons on wool growth was nearly immediate in another experiment (Hart et

al., 1963),

although the ambient temperature and pattern of photoperiod treatment differed. Hutchinson

(1965) demonstrated that the rhythm of wool

production in Southdown ewes was entrained to an accelerated photoperiodic rhythm with a lag of 2-3 months.

Maxwell

et al. (1988)

found that the frequency of the seasonal rhythm was increased to a cycle of approximately

8

months when sheep were exposed to continuous LD, but declined in continuous SD. In the extreme case of zero daylength (Hart,

1961), the seasonal wool growth rhythm was suppressed and

eventually disappeared. There is, however, one report of a short-term increase in winter and spring wool growth in response to SD (Hart,

1961).

In shedding breeds such as the New Zealand Wiltshire, a SD to LD transition induces a follicle growth cycle (Pearson

et al., 1996) resulting in a short-term inhibition

and a long-term stimulation of wool growth.

Despite some conflicting results, from the various experiments reviewed it seems that the annual rhythm of wool growth is entrained by photoperiod. The effects of photoperiod on wool growth appear to require a period of adaptation but if the photoperiodic stimulus is increased by bright lighting and rapid changes of day length, entrainment of the rhythm is more successful. Daylength is influenced by latitude as well as season. Sumner

et al. (1998)

recently reported that the wool growth of Romney ewes exposed to a longer summer daylength and a shorter winter daylength had a greater degree of seasonality.

1.2.4 Temperature

Long··woolled sheep breeds show a marked depression in wool growth in winter, although raising the ambient temperature during winter does not necessarily affect the rate of wool growth of well-fleeced animals (Coop, 1953). This study, and others (Morris,

1961),

has led to the conclusion that temperature plays a relatively small part in the wool growth cycle. However, the situation is not so clear in clipped areas of skin, as studies indicate that wool growth rate can differ from that of the fleece as a whole (Bennett

et al.,

1962;

Wodzicka-Tomaszewska & Bigham,

1968).

Bennett

et al. (1962),

Doney and Griffiths

(1967),

and Downes and Hutchinson

(1969)

all observed a reduction in length growth rate on closely clipped skin patches after short­ term exposure to cold. Similarly, Lyne et al. (1970) and Jolly and Lyne

( 1970)

showed that lowered subdermal temperatures reduce length growth rate but not diameter. The depression in wool growth becomes more pronounced as the degree of skin cooling intensifies (Jolly & Lyne,

1970).

Exposure to elevated temperatures produces the reverse effect on fibre diameter (Jolly & Lyne,

1970; Lyne et al., 1970) and either an increase (Jolly

& Lyne,

1 970) or no

effect on length growth rate (Lyne et al., 1 970).

It has also been demonstrated that wool growth rate was increased relative to uncovered patches when the midside patch was covered with a bag of wool to simulate fleece cover (Bennett et al., 1962; Wodzicka-Tomaszewska & Bigham,

1968).

When the entire sheep is exposed to cold by shearing, more wool is produced in the subsequent months (e.g. Sumner & Armstrong

(1987» .

The stimulation of wool growth rate following shearing is contrary to the generally observed depression in wool growth when a small area of skin is exposed to cold following clipping. Exposure of skin to cold reduces blood flow (Doney & Griffiths,

1967),

which is correlated with wool production (Hocking Edwards & Hynd,

1994),

and therefore nutrient supply to the exposed area. However, in the shorn sheep, the reduction in blood flow is likely to be compensated by an increase in feed intake (Elvidge & Coop, 1974).

1.2.5 Summary

Nutrition plays an important role in the control of wool growth, this effect being pronounced in the pregnant ewe. The need to meet the nutritional requirements for the growing fetus, mammary gland development, an increase in live weight during pregnancy and milk production during lactation, is partially at the expense of wool growth. Photoperiod and temperature both affect wool growth directly, and indirectly via their influence on pasture quantity and quality. Blood flow and skin temperature are also important physiological factors regulating local wool growth.

1 .3 CHANGES IN HORMONE CONCENTRATIONS DURING PREGNANCY AND LACTATION

1.3.1 Introduction

Endocrine changes resulting from the presence of an embryo in the uterus are central to the continuation of luteal function, to the maintenance of pregnancy and fetal development, and to the triggering of parturition and the onset of lactation (Cox,

1975).

Concentrations and balances of many hormones have been shown to change during gestation, and the establishment and maintenance of pregnancy requires interactions between the developing conceptus and the maternal system (Bazer & First,

1983).

The importance of the conceptus, as well as the mother, as a source of gestational hormones has long been recognised. During the last month of gestation, in addition to the secretions of the maternal pituitary and corpus luteum, the conceptus may also secrete hormones into the maternal system.

The following is a detailed account of the changes in plasma hormone concentrations during pregnancy and lactation in sheep from the literature. A variety of external factors have been shown to influence the levels of these hormones in the maternal circulation and these will also be discussed.

1.3.2 Progesterone

Progesterone is synthesised and secreted by luteal cells (Niswender et al.,

1985).

There is little evidence of any major increase in plasma progesterone concentration above approximately

2-5

ng/ mL during the first

50

days of gestation (Bassett et al.,

_1969;

_McNatty et al.,

_1972;

_Stabenfeldt et al.,

_1972;

Tsang,

1978;

Boulfekhar & Brudieux,

1980;

Sawyer,

1995).

This is consistent with the view that the corpus luteum is the main source of progesterone in the maternal circulation up to this time (Bassett et al.,

_{1969; Stabenfeldt}

et al.,

_1972;

Bassett & Thorburn, 1973).

From day

50

until day

120 of gestation, progesterone concentrations in the

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