<b>Impact of subclinical mastitis on milk yield and economic return of dairy cows</b>

Abstract

The general objectives of the present thesis were to evaluate: (i) the effects of subclinical mastitis (SM) caused by major pathogens on SCC, milk leukocyte differentials (MLD) and milk yield; (ii) milk yield losses caused by SM at the cow and quarter level; and (iii) the economic impact of SM caused by major pathogens. The thesis was structured in four studies. In study 1, quarter milk samples (n = 302) from 78 cows with SCC >200,000 cells/mL were analyzed by milk leukocyte differential (MLD) methodology and by microbiological culture (MC). Quarters with positive-culture results were obtained from 102/156 (65.4%) of MLD-positive milk samples, while 28/135 (20.7%) of MLD-negative milk samples were MC-positive. When MC was considered the gold standard for mastitis diagnosis, the sensitivity (Se) of the MLD was 65.4% (IC95% = 57.4 to 72.8%) and the specificity (Sp) was 79.3% (IC95% = 71.4% to 85.7%).In conclusion, the use of the MLD on cows with monthly composite SCC > 200×10 3 cells/mL for screening at quarter level identified quarters more likely to be culture-positive. In study 2, the effect of different pathogens was evaluated by comparison of contralateral (healthy and infected) mammary quarters of 146 lactating cows. The impact of SM on economic return (quarter milk yield × milk price) was determined by applying milk payment estimates on milk collected from healthy versus infected glands. The milk losses ranged from 0.07Kg/quarter.milking to 2.9 Kg/quarter.milking, and varied according to the pathogen causing In the period from 1990 to 1995 the average annual growth of milk production was 2.6%, between 1995 and 2000 it was 3.7% per year. Since 2015 the Brazilian economy has been experiencing difficulties due to an economic crisis. Dairy farming registered an increase in production costs, as well as a reduction in the number of milked cows and milk production. There was also a drop in the price of milk paid to the producer, as well as a contraction in product acquisition by industries and exports of dairy products (IBGE, 2015). Regarding demand, a continued decline in Gross Domestic Product (GDP) by 3.8% in 2016/2017 is expected, with the loss of income and employment. While inflation remains at high levels, leading to reduction in dairy product consumption, this decline will lead to decreased production and encourage producers to reduce their costs (CONAB, 2016 Consumption should grow at an annual rate of 1.9%, consistent with the country's milk production. However, placing consumption at a level slightly above national production ( Figure 1), a larger volume of milk may be required via imports, which would be close to 1.0 billion liters of milk by the year 2023, unless specific public policies for the dairy sector are implemented (BRASIL, 2013). The availability of milk per capita in Brazil is still low when compared to that of developed countries, where the purchasing power of the population is higher. The average consumption in these developed countries reaches 220 liters/inhabitant/year. To meet g...