Perennial ryegrass endophyte effects on plasma prolactin concentration in dairy cows

The aim of the two experiments reported here was to determine whether cows grazing perennial ryegrass infected with wild endophyte (Neotyphodium lolii) had reduced plasma prolactin concentrations compared with cows consuming a diet containing zero or low levels of ergovaline. The seasonal profile of plasma prolactin concentration was determined in 27 dairy cows grazing perennial ryegrass infected with wild endophyte, and in 28 cows fed a total mixed ration (TMR) diet in the 1999/2000 season. The concentration of prolactin in blood plasma from cows fed TMR was generally higher than the pasture-fed cows from November to April (20.6 versus 16.9 ng ml −1 , standard error of difference (SED) =1.13), but the magnitude of the difference changed over time. Peak ergovaline levels of 0.8 mg kg −1 were recorded in ryegrass pasture in mid March 2000. In a second experiment, 18 cows grazing perennial ryegrass infected with the novel endophyte AR1 (no ergovaline or lolitrem B production) had higher concentrations of prolactin in February 2001 than 18 cows grazing ryegrass infected with wild endophyte (25.2 versus 14.4 ng ml −1 , SED = 2.66), although concentrations were similar during January 2001. Prolactin concentration does not provide a reliable indicator of endophyte effects in dairy cows.