The summer -fall slump: Seasonal variation in average daily milk production in Prince Edward Island, Canada

Average test-day milk production in Prince Edward Island, Canada, was observed to follow a pronounced and consistent seasonal pattern, with peak and nadir production occurring in June and November, respectively. It was also observed that there was substantial herd-to-herd variation in the seasonal patterns of milk production, with some herds maintaining seasonally consistent average milk yields and others exhibiting a decline in average test-day milk yield of approximately fifty percent during the summer and fall months. There was also controversy surrounding the association of economic performance with seasonal variation in average test-day milk yield.;During the years 1990 to 1994, the average test-day production in November was 74.5% of that observed in June for all Prince Edward Island dairy herds. Herds tended to show similar patterns of milk production from one year to the next.;An income-over-feed-cost (IOFC) analysis was used to determine the relationship between seasonal variation in milk production and economic performance. A positive, linear relationship was found between seasonal patterns of production and IOFC, with seasonally consistent herds demonstrating higher IOFC than herds with marked seasonal variability in production. On average, for every 10 percent reduction in average test-day milk yield from June to November, IOFC of ;Delphi and conjoint analysis techniques were used to obtain estimates of the expected increase in pasture dry matter yield resulting from the use of various pasture management techniques. These two methods yielded results that were highly correlated when combined at the field level (R2 = .89) and that demonstrated good agreement with the appropriate data in the literature.;In-depth examinations of the relationship between nutrition, body condition score, internal parasite exposure and the seasonal pattern of milk production were completed before using multivariable modeling techniques to explain the inter-herd variability in seasonal variation in average test-day milk production in PEI. In addition, information on herd reproductive performance and herd management data were included in the multivariable models. The models explained a significant proportion of the between-herd variability in the seasonal patterns of milk production, and were found to be robust and reliable after thorough examination. A number of factors were found to be statistically associated with the seasonality of herd average test-day milk production. In the most parsimonious model these included herd level factors that measured the reproductive performance. (seasonal difference in days-in-milk), the internal parasite exposure levels (bulk tank milk Ostertagia ostertagi optical density values) and the nutritional management during the summer (kilograms of supplementary, non-forage DM cow-1 day-1 ). To rank the variables as to their relative importance, the regression coefficients were multiplied by the interquartile range of the observed values. Using this technique, reproductive performance and Ostertagia ostertagi optical density values were shown to have a similar impact on the seasonal pattern of milk production in a herd, whereas in absolute terms the daily amount of non-forage dry matter per cow had a marginally lesser effect. (Abstract shortened by UMI.).