Abstract
Abstract. The research studied a sample of 455 Polish Holstein-Friesian Black and White cows. Its aim was to apply and compare two modern statistical methods, i.e. classification trees and a logistic regression in examination of the impact of selected lactation-related factors (successive lactation, herd size and production level, year of calving, calving season, test day season, lactation phases and the amount of milk obtained in a test milking) on the somatic cell counts. Two different division criteria were taken into account in the creation of classification trees, i.e. entropy reduction and Gini coefficient. The quality of classification trees and multiple regression models was compared taking into consideration the following criteria: an average squared error, cumulative lift, Kolmogorov-Smirnov statistics and the area under the ROC curve. Having conducted the research, it may be concluded that from among the statistical methods applied, the best modelling of the level of somatic cell counts was obtained using the classification tree technique when the division criterion was based on the entropy function. According to the results of the study, somatic cell counts were diversified by the following factors, in a decreasing order of importance: herd production level, year of calving, subsequent lactation, calving season, day of test milking, herd size and the month used to take milk samples. Using somatic cell count as an udder health benchmark, it may be concluded that cows requiring particular attention as a result of udder diagnosis are from those in herds with high milk production levels, with individual cows producing up to 15 kg of milk.
Highlights
The reasons underlying variation in somatic cell counts (SCC) in cows’ milk are widely discussed in the literature and a plenty of research have been conducted to indicate factors affecting SCC level (Baltay 2002, Koç & Kizilkaya 2009, Król et al 2010)
The susceptibility of cows to mastitis is one of the more important functional problems of dairy cattle farming and one that affects the profit to be made from milk production
The frequency of mastitis increased for Holstein population due to genetic antagonism between milk production and clinical mastitis resistance (Gulyas & Ivancsics 2001)
Summary
The reasons underlying variation in somatic cell counts (SCC) in cows’ milk are widely discussed in the literature and a plenty of research have been conducted to indicate factors affecting SCC level (Baltay 2002, Koç & Kizilkaya 2009, Król et al 2010). A high level of somatic cells is associated with cows’ susceptibility to mastitis (Bradley 2002, Burston & Erskine 2003, Forsbäck 2010). This inflicts losses resulting from an early cows’ culling (Samoré et al 2003), lowered profitability of milk production and deteriorated quality for processing (Halasa et al 2007, Forsbäck 2010). The presence of somatic cells in cows’ milk can be indicative of udder health and milking hygiene, which translates into cows’ health and the quality of milk (Baltay 2002, Bradley 2002, Seegers et al 2003, Koç & Kizilkaya 2009, Skrzypek et al 2003)
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