Survey of milk composition in the Dublin liquid milk area.

The milk of a 10% stratified random sample of suppliers to the four main dairies in the Dublin liquid milk area was analysed monthly for chemical composition from December 1964 to November 1965. Average percentage values and respective standard errors for the 12 months were: total solids, 12.23? 0.03; fat, 3.63? 0.02; solids-not-fat (SNF), 8.60? 0.01; lactose, 4.46? 0.01; protein (cal culated), 3.19? 0.01. SNF levels were low, particularly in January, February and March, when the average monthly values for the samples examined were under 8.5%. Lactose content remained relatively constant throughout, and the decreases observed in SNF content were due largely to a reduction in protein content when the cows were indoors. Protein increased markedly when the cows went out on grass in April. Fat content continued to drop at this time and reached its minimum level in June (3.32%). The British Standard hydrometric method with the fat in the liquid state overestimated the SNF percentage by 0.11 on average compared with the gravimetric method. The discrepancy between the methods increased as the protein decreased and was at a minimum when the protein reached its highest value. INTRODUCTION The survey was initiated to investigate the composition ofthe milk supply in view of reports of low SNF levels at certain times of the year (1). Herd samples were taken monthly from December 1964 to November 1965. The accuracy ofthe British Standard hydrometric method with the fat in the liquid state, which is used in Ireland for the estimation ofthe SNF content in the milk supply, was also studied. EXPERIMENTAL Selection of suppliers The total number of suppliers to the four main dairies in Dublin during 1964 was 3,412, and they delivered approximately 80,000 gallons of milk per day. They were divided into eight categories according to the gallonage of milk they supplied during October, November and December 1963, and 10% were selected at random from each category. About 8 % of the suppliers were unavoidably missed on each occasion so that 316 suppliers on average were sampled monthly. 1 Present address: An Foras Taluntais, Horticulture and Forestry Division, Kinsealy, Malahide Co. Dublin 37 This content downloaded from 157.55. 39.58 on Thu, 25 Aug 2016 04:23:31 UTC All use subject to http://about.jstor. org/terms 38 IRISH JOURNAL OF AGRICULTURAL RESEARCH, VOL. 7, NO. 1, 1968 The geographical distribution of the suppliers selected in the statistical sample is shown in Fig. 1. Herd management data Information on herd breeds, calving dates and herd sizes was obtained for 1965 by a postal questionnaire to which 226 suppliers (66 % of the total selected) replied. Breed distribution was 68 % mixed herds, 15% predominantly (i.e., greater than 80%) Friesian, 14% predominantly Shorthorn, 2% Channel Island and 1 % Ayrshire. The calving pattern was found to be relatively uniform throughout the year as anticipated for this liquid milk area (Fig. 2). The percentage distribution of cows out on grass versus indoors is shown in Fig. 3: 85 % of the herds were brought indoors by November and the remainder in December. Fig. 3 also shows that 60% of the herds were put out on grass in April and 32% in May. The average size of herd was 21 cows, which is somewhat higher than the average (15 cows) calculated from the 1963 gallonage data referred to already. Sampling Each of the four dairies was sampled 1 day in 4 weeks, on a random schedule, to eliminate any bias that might occur from a set sampling routine. A sample of about