EFFECTS OF SEASON, HERD SIZE, AND GEOGRAPHIC REGION ON THE COMPOSITION AND QUALITY OF MILK IN THE NORTHEAST

Our objectives were to describe the milkshed comprising herds in New Y ork, western New Jersey, and central and eastern Pennsylvania in regar d to milk yield, composition, and quality and also to estimate the eff ects of season, herd size, and geographic area on those same variables . Data were collected fi om July 1993 through June 1994 from 3450 herd s. The effect of a somatic cell count (SCC) limit of 500,000/ml on mil k yield and the composition of monthly bulk tank milk for all marketed milk was estimated as was the frequency of deliveries of milk that co ntained SCC that were greater than this limit. All general linear mode ls for mean monthly yield of milk and milk components (fat and protein ) and SCC were significant for fixed effects of month and herd size wi thin quartiles for herd size (defined by the number of lactating cows) and significant absorbed effects of herds within quartiles for herd s ize within subregion. Milk yield, milk components (kilograms), true pr otein percentage, and SCC were significantly higher in spring than in fall for both data files (complete data file and data file containing only herds with SCC < 500,000/ml). Thirty-five percent of herds with < 27 lactating cows but only 15.3% of herds with > 62 lactating cows ha d greater than or equal to 1 mo with an SCC > 500,000/ml. For herds in the subregions, percentages of shipments with an SCC greater than or equal to 500,000/ml ranged from 10.5 to 20.2%. Herds with < 27 lactati ng cows contributed to the milkshed a disproportionate percentage of S CC (11%) compared with their percentage of contribution of milk (5%).