Responses to selection in ryegrass staggers lines of sheep

摘要

Ryegrass Staggers (RGS) is predominantly a summer/autumn metabolic disorder in ruminants, caused by ingestion of the lolitrem-B toxin from endophyte-infected perennial ryegrass, and it is common in New Zealand. It causes lack of neuromuscular co-ordination in susceptible animals under stress, e.g. when mustered by sheep dogs. It is of welfare concern, and it is costly to farmers because it severely compromises grazing management. A flock was established at AgResearch in 1993, with two lines bred forresistance (RGS-R) or susceptibility (RGS-S) to RGS. Lambs are scored for susceptibility each year, when grazing on high endophyte pastures, and mustered by sheep dogs. In 2005 and 2006, 94% and 91% of lambs respectively in the RGS-S line showed clinicalstaggers, when grazing together with the RGS-R line which had 2% and 6.5% of clinical cases, respectively (P < 0.001). Using the latest standardised RGS scoring system, the heritability estimate of resistance/susceptibility to RGS was 0.36 ± 0.04. To understand more about why and how the RGS lines differ, blood samples were taken from the breeding-ewe flock (n = 126) in June 2005, outside the usual RGS and facial eczema season. Compared with the RGS-R line, the RGS-S line ewes had a 23% higher activity (P < 0.001) of aspartate transaminase (AST), an enzyme indicating hepatic or muscular damage; there were similar findings in ewes sampled four weeks apart (between-animal repeatability for AST = 0.85). In plasma samples from selection-line yearling females (two birth years; n = 95), the RGS-S line had 27% and 37% higher AST and alanine transaminase activities, respectively (both P < 0.001), and a 38% higher creatine kinase activity (P < 0.05) than the RGS-R line. One explanation of these results is that, compared with the RGS-R line, selection in the RGS-S line may have changed two functions, a reduced ability to detoxify lolitrem-B, and an increased sensitivity to stress expressed as a tendency to show increased muscle tetany and damage.