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Association between blood β-hydroxybutyrate at 7 days postpartum and milk yield, disease occurrence and fertility in grazing dairy cattle with seasonal calving: a case study

Context Ketosis in grazing cattle has been sparsely studied. A large commercial grazing dairy in southern Chile, representative of a significant proportion of the systems in the country, was used in this case study. Aims The study had three objectives: (i) to establish a cut-off for β-hydroxybutyrate (BHB) concentration for subclinical ketosis (SCK), and use this to measure the proportion of cows with SCK at 7 days postpartum in spring- and autumn-calving cows; (ii) to describe the relationship of SCK and other periparturient diseases and fertility; and (iii) to compare milk yield of healthy cows and those affected by SCK in a dairy herd with autumn and spring parturitions under grazing conditions in southern Chile. Methods During 2016, 234 cows with autumn parturitions and 632 cows with spring parturitions (n = 866) were assessed for blood BHB at 7 days postpartum. A receiver operating characteristic analysis for a BHB cut-off value was completed. Models were developed for disease occurrence, culling risk, conception risk and pregnancy rate, considering SCK as the main explanatory variable. Key results In total, 810 cows were used for the final analysis. The frequency of cows with SCK, based on the cut-off value obtained (BHB ≥1.1 mmol/L), was 22.2% at 7 days postpartum. The risk of SCK was higher (P < 0.0001) in cows calving in spring (27.0%) than in autumn (10.3%), and in multiparous (24.6%) than primiparous cows (15.1%). The seasonal difference in proportion of cows with SCK was parity-dependent, because the frequency of SCK in multiparous cows was higher (P < 0.0005) in spring (32.0%) than autumn (10.1%), whereas SCK in primiparous cows showed no significant (P = 0.41) difference between spring (15.4%) and autumn (12.5%). Milk production up to 100 days-in-milk was greater (P = 0.002) in cows with SCK (3394 kg) than without SCK (3015 kg). Disease occurrence was higher (P < 0.0001) in cows with SCK and in multiparous cows (P < 0.0001). There was no difference in conception risk at first service (P = 0.62) or in overall pregnancy rate (P = 0.90) between cows with and without SCK. Conclusions Multiparous cows calving in spring had the highest risk of SCK (BHB ≥1.1 mmol/L). SCK was associated with higher milk yield and greater occurrence of other diseases, but not with reproductive performance. Implications Grazing herds have challenges with SCK that may require different management strategies depending on the calving season and the parity of the animals.