Effects of injectable trace mineral supplementation in lactating dairy cows with elevated somatic cell counts.

Objectives of this clinical trial were to evaluate the effects of injectable trace mineral supplementation (ITMS) on somatic cell count (SCC), linear score (LS), milk yield, milk fat and protein contents, subclinical mastitis cure, and incidence of clinical mastitis in cows with elevated SCC. Holstein cows from a commercial dairy farm in New York were evaluated for subclinical mastitis, defined as SCC ≥200×103 cells/mL on the test day preceding enrollment. Cows with a history of treatment for clinical mastitis in the current lactation and those pregnant for more than 150d were not eligible for enrollment. Cows fitting inclusion criteria were randomly allocated to 1 of 2 treatment groups. Cows assigned to ITMS (n=306) received 1 subcutaneous injection containing zinc (300mg), manganese (50mg), selenium (25mg), and copper (75mg) at enrollment (d 0). Control cows (CTRL; n=314) received 1 subcutaneous injection of sterile saline solution. Following treatment, visual assessment of milk was performed daily, and cows with abnormal milk (i.e., presence of flakes, clots, or serous milk) were diagnosed with clinical mastitis (CM). Chronic clinical mastitis was defined as cows with 3 or more cases of CM. Milk yield, milk fat and protein contents, SCC, and LS were evaluated once monthly. Additionally, randomly selected animals were sampled to test serum concentrations of selected minerals on d0 and 30 (n=30 cows/treatment). Treatment did not affect serum concentrations of calcium, magnesium, phosphorus, potassium, copper, iron, manganese, selenium, and zinc on d30. Injectable supplementation with trace minerals did not improve overall cure of subclinical mastitis (CTRL=42.8 vs. ITMS=46.5%), although a tendency was observed in cows with 3 or more lactations (CTRL=27.1 vs. ITMS=40.0%). Supplementation did not reduce treatment incidence of CM (CTRL=48.2 vs. ITMS=41.7%); however, it tended to reduce the proportion of cows diagnosed with chronic CM (CTRL=16.9 vs. ITMS=12.0%), particularly among first-lactation cows (CTRL=18.4 vs. ITMS=7.6%). Cure of subclinical mastitis was associated with higher serum concentrations of phosphorus and selenium on d30. Supplementing trace minerals to cows with elevated SCC had no effect on milk yield, milk fat and protein contents, SCC, and LS.