Loss in milk yield and related composition changes resulting from different levels of somatic cell count in Valle del Belice dairy sheep

Abstract

Mastitis is the most common and costly disease in the dairy industry and is responsible for several production effects. Mammary infections cause high somatic cell count (SCC) and severe damage to udder tissue and result in important losses of milk yield and quality, curd and cheese yields in dairy ewes. The aim of this study was to determine the effect of different levels of SCC on milk yield and composition in Valle del Belice dairy sheep. The original data set used for this study included 92,261 records of 6,763 ewes. Data were collected between 1994 and 2006 in 15 flocks. Test-day records of milk yield (MY), fat % (F%), protein % (P%), and SCC were collected following an A4 recording scheme. Records with incomplete information were deleted. Moreover, records with less than three test-day measurements within lactation for milk traits were discarded. After editing, the dataset consisted of 17,060 observations on 2,418 ewes. To evaluate loss in test day MY, F% and P%, five different classes of SCC were arbitrarily defined (1: SCC ≤500,000; 2: 500,000< SCC ≤1,000,000; 3: 1,000,000< SCC ≤1,500,000; 4: 1,500,000< SCC ≤2,000,000; 5: SCC >2,000,000). A linear model as factorial design of fixed effects was used with the GLM procedure of S.A.S. statistical package. The following model was used: y=FK+OP+AOP+STG+SCC+DIM+e, where y is the response variable measured (MY, F% or P%), FK is the effect of flock (15 levels), OP is the effect of parity class (4 levels), the covariate AOP is the age of lambing, STG is the season of lambing (3 levels), SCC is the fixed effect of somatic cells count class (5 levels), DIM is the class of days in milk (5 levels) and e is the random error. All fixed effects included in the model were significant (P<0.001). The estimated losses in MY according to the level of SCC used were approximately 20%, (903g for SCC ≤500,000 and 726g for SCC > 2,000,000). However we had an increase of 4.9% and 2.7% on F% and P% respectively. In particular for F%: 5.75% for SCC ≤500,000 and 6.05% for SCC >2,000,000; for P%: 7.43% for SCC ≤500,000 and 7.64% for SCC >2,000,000. Results suggest the need for the establishment of prevention and control programs of mastitis based on SCC routine monitoring in dairy sheep herds. These values could be suitable for use as basic input in calculations of economic losses due to mastitis.