Is there a specific weakness in staple strength around the break of season?

In a Mediterranean climate the weak point of the staple normally occurs at the break of season in autumn, but it is not clear whether the staple simply breaks at the minimum fibre diameter or whether there is a specific weakness at this time. Three hypotheses were tested to determine if specific environmental effects on staple strength could be detected under field conditions. First, environmental stresses associated with rainfall and low temperature, resulting in a sudden disruption of warm summer–autumn conditions and lack of feed, may cause follicle shutdown. Second, the sudden decline in feed available following the rain event and the response of sheep to chase the green pick rather than eat the available dry feed may reduce the flow of nutrients to wool. Third, the slow adaptation by rumen microorganisms to changes in the diet from dry to green pasture may result in a further reduction of nutrients available to the wool follicle. We compared a group of sheep managed to minimise nutrient intake fluctuations at the break of season with a group grazing under normal farm practice and tested the hypothesis that a specific lowering of staple strength is associated with the break of season. On 2 April (3 days after the first rains of the season), 120, 18-month-old Merino wethers were allocated to 2 groups, paddock and yard. The paddock group comprised sheep following normal farm practice. The yard group was confined to yards and fed to maintain liveweight, to determine whether the break in the staple was associated with the rainfall event or with the subsequent changes in feed intake. These animals were left off feed for 4 days beginning 24 April. Within each group, 2 separate treatments were imposed. The paddock group was split, and half were moved into covered pens inside a shearing shed on 15 May, a few days before a second major rainfall event, to examine directly stresses associated with rainfall. The sheep from the yard group were kept as a single mob until 3 June, when they were split into 2 groups, sudden and gradual, in relation to their release onto green pasture, to examine the effect of adaptation time to green feed on the flow of nutrients to wool and staple strength. The sudden group was released onto green pasture, while the gradual group was given access to the pasture for increasing periods over the next week. The point of break was delayed by moving sheep into yards after the initial rainfall, suggesting that the rain event per se was not the direct cause of the break in the staple in this experiment. Furthermore, protecting the paddock sheep against the second major rainfall event by shedding half of them did not affect staple strength. The point of break in the staple in the yard group occurred after an accidental 4-day feed deprivation period. This indicates that even short periods of liveweight loss from feed deprivation due to poor or inattentive management when sheep are in low nutritional condition at this time of the year may precipitate the point of break. The rate of release onto green feed after yarding did not significantly affect staple strength and we conclude this was not an important factor. We conclude that neither stress associated with rainfall nor adaptation of ruminal microorganisms to the change in feed weakened the wool.