Genetic parameters for milk yield, contents of fat, total protein, casein and serum protein, individual laboratory cheese yield, and somatic cell counts (SCC) were estimated from 7492 monthly test-day records of 1119 Churra ewes. Estimates were from multivariate REML using analytical gradients (AG-REML) procedures. Except for fat content, estimates for the other routinely recorded traits (milk yield, protein content, and SCC) agreed with those previously obtained in this and other dairy sheep populations. Protein content and composition had the highest heritabilities and repeatabilities. Heritabilities for protein and casein contents were very similar (0.23 and 0.21, respectively), and genetic correlation between the traits was close to unity (0.99). Accordingly, casein content is not advisable as an alternative to protein content as a selection criterion in dairy ewes; it does not have any compelling advantages and costs more to measure. Individual laboratory cheese yield (ILCY) obtained with Churra ewes had a low heritability (0.08), suggesting that potential for selection for this parameter would be possible but is not recommended. All correlations with ILCY were high and positive except for milk yield. A high SCC was accompanied by an increase in serum protein content and involved a loss in milk yield.
A total of 7492 test-day observations for mean contents of fat, protein, casein, serum protein and lactose and individual laboratory cheese yield (ILCY) were obtained, at approximately monthly intervals, from 1119 ewes belonging to eight Churra dairy flocks. The effect of various factors on these variables was examined and phenotypic correlations among all traits were estimated. Least squares analyses showed significant effects of flock test-date, stage of lactation, age of ewe, and number of lambs weaned on almost all variables. Protein content and composition were not affected by the number of lambs weaned. ILCY had an unadjusted mean (26·55 kg cheese/100 l milk) close to those reported for real cheese yield in dairy ewes and was affected similarly to the main milk components. Fat, protein, casein, and serum protein contents, and ILCY, showed a generally increasing trend as lactation progressed. These components reached a minimum at 1 month into lactation, when milk yield was highest, and increased for the remainder of the lactation. ILCY depended mainly on fat, protein and casein contents. Protein and casein contents were closely related and equally correlated with ILCY. An increase in somatic cell count (SCC) was associated with decreased milk yield and decreased lactose content.
-The effects of some environmental variation factors and the genetic parameters for total milk traits (fat content, protein content, casein content, serum protein content, lactation mean of individual laboratory cheese yield (LILCY), lactation mean of somatic cell count (LSCC), and milk yield) were estimated from the records of 1 111 Churra ewes. Genetic parameters were estimated by multivariate REML. Heritability for fat content was low (0.10) as is usually found in the Churra breed. Heritabilities for protein content, casein content, serum protein content, LILCY, milk yield and somatic cell count were 0.31, 0.30, 0.22, 0.09, 0.26 and 0.11, respectively. The highest heritability estimates were for protein and casein contents. Casein content is not advisable as an alternative to protein content as a selection criterion for cheese yield improvement; it does not have any compelling advantages and its measurement is costly. Our results for LSCC indicated that efforts should focus on improving the level of management rather than selecting for somatic cells, in the actual conditions of the Churra breed.
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