Effects of oyster glycogen intramammary challenge on primiparous cow milk somatic cell counts, milk yields, and milk composition

Mastitis is a common and costly disease in the dairy industry that reduces milk production in affected mammary glands. The local mechanisms that result in reduced milk production of affected mammary glands are incompletely understood; elucidation of these mechanisms is dependent on the use of hypothesis testing studies, but few experimental models exist. The objective of this study was to develop a mastitis challenge model, using a split udder design, to reduce milk yields by approximately 15% in udder halves challenged with oyster glycogen, a known inducer of leukocyte recruitment, relative to udder halves treated with saline. Four primiparous Holstein cows in mid lactation were used. One udder half of each cow was randomly selected and challenged with oyster glycogen (OYGLN), and the opposite udder half was treated with saline (SAL). Milk yields and components were measured at each milking (3×/d) for 3 d postchallenge. No signs of clinical mastitis were observed. Milk somatic cell scores, yields, and components were similar between OYGLN and SAL udder halves at time of challenge. Milk somatic cell scores markedly increased in OYGLN halves postchallenge and were greater than SAL halves for the duration of the trial. Lactose concentrations of OYGLN udder halves were transiently lower than in SAL udder halves, but protein concentrations were greater at 2 milkings postchallenge in OYGLN halves. Milk yields and energy-corrected milk yields did not differ between OYGLN and SAL udder halves overall, nor at any postchallenge milking. A single intramammary challenge of oyster glycogen was unsuccessful in eliciting a disparity in milk yields between challenged and saline control udder halves despite the marked leukocyte infiltration observed in the former. These results indicate an incomplete understanding of how milk yields are reduced in mammary glands affected by subclinical mastitis and that transient somatic cell recruitment and infiltration alone do not directly reduce milk yields during subclinical mastitis.