The experiment was conducted in an extensive breeding of dairy cows in Ghineh-Mount Lebanon in order to identify the effect of zinc methionine supplementation on milk production affected by subclinical mastitis. The animals were divided into 2 groups; the first group was non supplemented (control ZM-, n=7), while the second was supplemented with 4 g the first month (June) before doubling the dose to 8 g of Zn-methionine/head/day the next month (July) (experimental ZM+, n=7). Milk samples were collected once a week from 40 teats selected based on CMT score during the two months June and July. The methods employed in this study included California mastitis test, bacteriological culture and milk composition analysis for milk fat, protein and lactose content. Milk yield and milk composition decreased significantly (P < 0.05) with increasing of total aerobic bacterial count TABC. Zinc methionine ZM supplementation for subclinical mastitis cows resulted in significant decrease in total aerobic bacterial count comparing to time effect (from 1.16 ± 0.20, day1 to 0.70 ± 0.13 x105cfu/ml, day 57; P < 0.001). The obtained results showed a significant increase in milk production in the supplemented group ZM+(16,00 ± 0.80, day1 to 18.86 ± 0.71 l/day, day 57; P < 0.001). Milk lactose and protein content were also significantly increased especially after one month of treatment in ZM+ group (from 4.10 ± 0.06 day 1, to 4.40 ± 0.04 % day 57 for lactose content and 2.70 ± 0.08 day 1, to 3.20 ± 0.05 % day 57 for protein content; p<0.001). Whereas, no significant difference was noted in milk fat content between groups (P> 0.05). This study showed the positive effect of supplementation of zinc methionine on TABC, milk yield and milk composition. This shows the importance of chelated minerals supplementation in the ration, in order to prevent the progression of subclinical mastitis to clinical cases that cause economical loses in dairy cows and affects the composition of the milk and consequently will be refused by the industries.
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