Milk yield and composition of cows with subclinical mastitis caused by environmental Streptococcus

Abstract

The aim of this study is to evaluate the effect of bovine subclinical mastitis caused by environmental Streptococcus on milk yield and quality at mammary quarters level, considering its high prevalence in dairy herds and the absence of specific control measures for these agents. The specific objectives are to evaluate the effect of bovine subclinical mastitis caused by Streptococcus uberis, Streptococcus bovis and Streptococcus dysgalactiae on: a) somatic cell count (SCC); b) the crude protein content and fat; c) the milk yield; and d) the milk liter price. In this study six dairy herds will be enrolled (estimate, 600 dairy cows) and; after lactating cows will be selected (estimated 60) with subclinical mastitis caused by environmental Streptococcus (S. uberis, S. bovis and S. dysgalactiae). Representative milk samples of complete milking at mammary quarter level of cows selected will be collected, totaling 240 mammary quarters. Concerning the results, 12 cows with positive isolation of each agent will be randomly selected; taking into account the number and stage of lactation. The bacterial isolates will be identified to the species level by conventional microbiological culture. The crude protein content and fat will be analyzed by infrared absorption and CCS will be determined by flow cytometry. To determine the effect of subclinical mastitis caused by environmental Streptococcus on liter milk price (LMP), the Brazilian average price based on the average of the last 20 years adjusted for inflation will be used. After that, the amount in accordance with the level of CCS and crude protein content and fat will be added to LMP; this amount will be distributed in bonus tracks, neutrality and penalty like a payment system for milk quality. LMP equations of mammary quarters affected by S. uberis, S. bovis and S. dysgalactiae will be determined by binary regression. Additionally, it will be performed the DIF of SAS® procedure between the means of LMP assigned to the healthy and infected quarters to determine the effect of subclinical mastitis caused by S. uberis, S. bovis and S. dysgalactiae concerning milk quality payment. The effect of subclinical mastitis caused by environmental Streptococcus at level group and species by mammary glands on the SCC, composition and milk yield will be assessed, using variance analysis by randomized block design. It is expected with the present study to estimate the impact of bovine subclinical mastitis caused by environmental Streptococcus, at mammary quarters level, on milk yield and quality. Thus, this study may also serve as a basis for future research on specific control strategies for these agents, such as the evaluation of effectiveness and costs of mastitis treatments caused by environmental Streptococcus.