Fleece growth in Australian cashmere goats. IV* The role of prolactin in the initiation and cessation of cashmere growth

An experiment was designed to investigate the effect of prolactin (Prl) on the initiation and maintenance of cashmere growth. Intra-muscular injections of bromocryptine (0.5 mg/kg) three times per week were used to suppress Prl secretion from August to December or from January to June in two groups of three male and three female goats. Bromocryptine effectively suppressed Prl concentrations to below 10 ng/mL for the duration of the treatments. Initiation of cashmere growth goats injected in July to December occurred in January after high post-treatment levels of Prl began to decline. Cashmere growth of male control goats was considered unrepresentative of normal cashmere growth, but initiation in female control goats occurred in December. Bromocryptine treatment from January to June had no effect on active cashmere growth. but inhibited cashmere growth not already initiated. Bromocryptine injections had no effect on serum T4 concentrations. The results are discussed in relation to the role of Prl in the initiation, maintenance and cessation of cashmere growth. It was concluded that either natural or induced peaks of Prl concentrations were associated with the initiation (falling concentrations) and possibly also with the cessation of cashmere growth (rising concentrations)

Measurement of the components of the cashmere growth cycle in Australian cashmere goats

Seasonal growth and shedding of cashmere fibre makes total fleece weight measurements difficult to interpret in relation to environmental and experimental variables. In studies of the effects of nutrition and photoperiod on cashmere growth, a technique has been developed which provides statistically analysable values for growth rates, maximum fibre length, length of growing period, and for dates of initiation and cessation of cashmere fibre growth. This method requires the measurement of cashmere length at monthly intervals at three sites (shoulder, midside and hip) on the goat. The model assumes that once initiated, the rate of cashmere length increase was linear until cessation, and that the initiation of growth occurs at a fixed time controlled by photoperiod. This paper provides a method for quantitative estimation of the dates of initiation and cessation of fibre growth, and for determining the linear rates of cashmere and hair fibre growth. The method involves the measurement of cashmere fibre length at 28 day intervals over the period of growth, and calculation of rate of cashmere growth from the linear portion of the graph relating length and date of sampling. Extrapolation of this line to time 0 provides an estimate of initiation date. Cessation date is then calculated as the date where maximum length observed intersects this linear plot, and the period of growth estimated as the difference between initiation and cessation dates. This simple model was found to closely fit observed data from individuals or groups of goats and, with the appropriate measures of variation, permits statistical comparisons of these values between individuals or groups

Fleece Growth in Australian Cashmere Goats .1. the Effects of Nutrition and Age On Fleece Growth, Prolactin and Thyroxine Concentration

Australian cashmere goats were studied from January to July to assess the effects of increasing feeding level (maintenance, M: 42 g DM/W0.75/day, 1.4 M, 1.8 M and 2.3 M (ad libitum)) and age (Young, 6 months; Old 18 months) on fleece growth, and plasma concentrations of prolactin (Prl) and thyroxine (T4). Comparisons of fleece composition (weight, cashmere fibre diameter, proportion of cashmere) were made from fleece shorn monthly (left side) and at the end of the experiment (right side). There was no significant (P > 0.05) effect of feeding level or age on cashmere weight (39.8 g), cashmere diameter (15.3 mum) or hair weight (224.7 g) at shearing in July. Similar results were observed with monthly fleece composition studies, although cumulative cashmere weight for Young goats (27.1 g) was significantly (P < 0.05) less than that of Old goats (48.5 g), and average fibre diameter of cashmere of Young goats (13.6 mum) was less than that of Old goats (15.6 mum). Serum Prl concentration in groups on higher feeding levels (1.8 M, 2.3 M) increased significantly (P < 0.05) over the duration of the study (45 to 70 ng/mL and 15 to 70 ng/mL for 1.8 M and 2.3 M respectively), whilst that of the groups on lower feeding levels (M, 1-4 M) remained low (15 ng/mL). The results are discussed in relation to the seasonal change in fleece composition and hormone secretion for the different feeding levels and age groups, and a possible antagonism between Prl secretion and fleece growth