Abstract
The study was designed to determine the relation between β-hydroxybutyrate (BHB) concentrations in milk from dairy cows after calving and the length of the previous lactation, the dry period and the 305-day normalized production, and to assess the influence of BHB concentrations on culling and test-day milk productions and somatic cell counts (SCC) throughout the lactation that followed the BHB measurement. The data used in the study were obtained from 59187 cows in the Galicia region (Spain). BHB determination was performed using Fourier-transformed infrared spectrometry from the milk samples collected from each cow on the first post-partumtest day. For statistical analysis, the following methods were applied: (i) ordinal regression to assess the effect of the length of the previous lactation, the dry period and the 305-day normalized milk production on milk BHB, (ii) a Cox model to estimate the influence of the BHB concentration on risk of culling (overall and for a variety of reasons) and (iii) linear regression to assess the link between BHB and the milk yield and SCC obtained from each of the tests day performed throughout lactation. The probability of having higher BHB concentrations increased when the length of the previous lactation (p=0.006), the dry period (p=0.003) and the 305-day normalized milk yield (p=0.005) increased. However, the slight increase observed (especially for the case of the dry period and the 305-day milk yield) would not justify that measures be implemented to reduce these traits. Higher concentrations of BHB led to an increased risk of culling due to 'death' (p≤0.001) and 'urgent slaughter' (p≤0.002) (both causes of involuntary culling). It also led to a reduction in milk production (p<0.001) and an increase in SCC (p<0.001) in the post-partum; from that moment onward (including peak lactation), there were no differences in those two parameters depending on the BHB levels.
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