Animals and experimental design
This experiment was performed at the YiWei White Cashmere Goat Farm located in the Inner Mongolia Autonomous Region of China (latitude 39°06’N, longitude 107°59’E and at an altitude of 1500 m) from April 30, 2013, to April 30, 2014. All procedures used in this study were approved by the Animal Care and Use Committee of China Agricultural University (Beijing, China).
Eighteen 2.1-year-old lactating, female, half-sib, Inner Mongolian cashmere goats with an initial body weight of 32.92 ± 2.78 kg were combed with a small wooden rake to remove the previous year’s fleece. The goats were then randomly assigned to two groups (n = 9) including a control and a treatment where melatonin (2 mg/kg BW; Beijing Kangtai Biological Technology Company, Beijing, China) was implanted on April 30 and June 29. Melatonin was subcutaneously implanted at the base of the ear and the dose applied was based on the work of Yue et al. .
During the experimental period, all goats were maintained under natural photoperiodic conditions and the feeding and management was typical of other goats at the cashmere goat farm where goats are kept year round in a desert pasture with occasional supplementary feeding. Goats were provided with grazing and supplementary feeding management from January to June, with 0.275 kg/d concentrate (70 % corn and 30 % condensed feed purchased from Baotou Jiuzhoudadi Biotech Company, Baotou, China) per goat provided in January, 2013 which was gradually increased to 0.4 kg/d in April and subsequently increased to 0.55 kg/d in May and June to meet the needs of the goats during lactation. Goats were grazed on natural pasture without supplementary feeding from July to December. The composition of the primary vegetation in the area was Caragana stenophylla poiark, Caragana rorsninskii kom, Agriopyron cristutum gaertn, Agriopyron cristutum schut, Alium polyrhizum turcz, Artemisia frigidi willd, Artemisia ordosica praschen, Stipa breviflora griseb, Haloxylon ammodendron bunge, some of which are grazed only by goats . Mating occurred over approximately 30–60 days (October-November, 2013) with parturition during the months of March-April, 2014.
Data and sample collection
A patch of fleece (30 mm × 30 mm) was shorn at the end of every month from the mid-side of each goat at the skin level. Each clipping was obtained immediately adjacent to the location of the last shearing but was always different from any previously sampled areas. Samples were manually separated into cashmere fibre and guard hair samples. The cashmere fibre samples were stored in sealed polythene bags at room temperature for subsequent analyses of fibre length and diameter.
All goats were individually identified. The body weight of each goat was recorded at the beginning and end of the study. Identification number, kidding date, type of birth, sex, and birth weight were also recorded. Cashmere was harvested at the end of April, 2014 by combing as previously described and was weighed using an electronic scale following harvest.
The cashmere fibre samples were soaked in carbon tetrachloride detergent solution overnight, rinsed thoroughly in deionized water and then dried at 80 °C. The stretched length of the cashmere fibre was measured to the nearest millimeter and the amount of cashmere growth was calculated. The measurements were conducted each month on 100 grab samples randomly selected from the separated cashmere fibres. The diameter of 200 randomly chosen fibre samples was measured using an Optic Fibre Diameter Analyzer (CU-6, Beijing United Vision Technical Company, Beijing, China).
Average cashmere growth rate was determined from the linear portion of cumulative cashmere growth curves, in which a regression line (Y = A + bX where Y is the fibre length and X is the day of the year) was calculated to develop a model that could explain the cycle of cashmere growth . The slope of the regression line (b) describes the average rate of fibre elongation. The initiation date of cashmere growth was determined from the regression equation relating cashmere growth and time as the point of intersection with the x-axis (Y
min = 0). The cessation date was defined as the time when maximum length was achieved, and calculated as the point where Y
max intersected the regression line relating change in cashmere length and date.
The data were analyzed using the t-test procedure of SAS . Statistical differences between initiation and cessation date, rate and phase of cashmere growth were detected by t-tests, using estimates of variance described elsewhere .