Increased somatic cell counts (SCC) are associated with reduced milk yield. Additionally, it has been hypothesized that as milk yield increases, SCC is diluted in cattle without an intramammary infection (IMI). If the hypothesis is correct, estimates of SCC from high-yielding cattle without an IMI are likely to be lower than those from low-yielding cattle without an IMI. The objectives of this paper were to investigate the presence of a potential dilution effect between SCC and milk yield, overall and by parity, and to estimate lactation milk production losses with increasing SCC in Irish dairy cattle. The data consisted of 100 randomly selected herds from all milk recording herds between 2008 and 2010. The data set comprised 8,229 cows, of which approximately 90% were Holstein or Holstein crossbred animals. Various adjustments were used to investigate the presence of a potential dilution effect between SCC and milk yield; additionally, lactation milk production losses with increasing SCC and parity were estimated. The data had an inherent hierarchical structure, with lactations nested within cows and cows within herds; thus, a linear mixed model with 2 random effects was used. We found no evidence of a dilution effect of SCC with increasing milk yield in Irish dairy cattle. Average milk production losses were estimated, and they increased with increasing SCC compared with the referent of ≤50,000 cells/mL. Additionally, for all SCC values for parities 1 to 3, this production loss increased significantly with increasing parity. Estimated milk losses for parities 4 and 5 did not differ, and differences between parities 3 and 4 were significant only for SCC values <300,000 cells/mL. The estimated milk loss with increasing SCC varies greatly across studies, with the results from the current study exceeding most previously published results (except for results from the UK). Several factors could explain these differences, including geographic factors such as milk yield and predominant mastitis pathogens. The dilution effect warrants further work, as does the effect of prior duration of increased SCC on milk yield and the potential for compensation of milk yield losses over a lactation.
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