The aim of this study was to determine risk factors associated with milk fever (MF) occurrence in dairy cows in Costa Rica. A total of 69,870 cows from 127 dairy herds were included in the study. Data were collected in the VAMPP (Veterinary Automated Management and Production Control Program) software by the Population Medicine Research Program of the Veterinary Medicine School, National University of Costa Rica; from 1985 to 2014. To determine the risk factors, 2 logistic regression mixed models were evaluated. The first model used breed (B), month of calving (M), quinquennium of calving (Y) ecological life zones (Z) and calving number (N) as fixed effects. The second model excluded first lactation animals and cows without productive information, had the same fixed effects of the first model plus: previous MF case (C), previous lactation length (L), previous dry period length (D), previous corrected 305d milk yield (P), and calving interval length (I) as fixed effects. Both models used animal (a) and herd (h) as random effects. Of the 235,971 recorded lactations, 4,312 (1.83%) reported MF event. The significantly associated (P < 0.05) risk factors for MF occurrence, ranked by their highest odds ratio (OR) are listed on Table 1. The findings of this study are the first data reported for a population study on risk factors for MF in Costa Rica. Some of these results might be used to improve preventive management practices at the farms to reduce the incidence of this metabolic disease. Table 1. Risk factors associated with milk fever (MF) occurrence ranked by odds ratio (OR) in Costa Rican dairy cattle¹

Risk factor		Riskiest category		Reference category	OR	95% CI
Calving number		6th or more calving		1st calving	52.40	40.94–67.08
Breed		Jersey		Brown Swiss	2.88	1.66–4.98
Previous MF case		Previous case reported		No previous case reported	2.39	2.17–2.63
Quinquennium of calving		1990-1994		2005-2009	2.33	2.00–2.78
Month of calving		July		December	1.37	1.16–1.61
Previous 305-d milk yield		1,000 kg over µ		µ for 305-d milk yield	1.17	1.14–1.21
Previous lactation length		30 d over µ		µ for lactation length	1.06	1.03–1.09

¹µ = population mean; 6,021 kg for previous 305-d milk yield, 316 d for previous lactation length.