Effects of season and herd milk volume on somatic cell counts of Florida dairy farms

Dairy farms in Florida produce less milk and milk with higher somatic cell counts (SCC) in the hot and humid summer. This has consequences for the interpretation of average milk quality. The objectives were to describe the associations of bulk tank SCC (BTSCC) with time of the year and the milk volume per farm. Monthly BTSCC and milk volume records from 84% (in 2012; n = 1,308) and 77% (in 2013; n = 1,200) of the 130 dairy farms in Florida were used. Data were analyzed separately per year. We calculated arithmetic averages of the BTSCC for each farm (ASCCf), each month (ASCCm), and each year (ASCCy). We used the milk volume to calculate a milk-weighted average for each farm (WSCCf), each month (WSCCm), and each year (WSCCy). Period 1 (P1) was defined as February, March, and April, and period 2 (P2) was defined as August, September, and October. These periods generally had the lowest and highest BTSCC throughout the year, respectively. Seasonality was expressed by the P2/P1 ratios of BTSCC and milk volume in both periods. In 2012 and 2013, 72 and 74% of the monthly milk volume observations were < 400,000 cells/mL. A clear seasonal pattern with lower milk volume and higher ASCCm during P2 was observed for most farms. The averages of the P2/P1 ratio of milk volume were 0.68 and 0.74 in 2012 and 2013. The averages of the P2/P1 ratio of SCC were 1.30 and 1.65 for 2012 and 2013, respectively. The WSCCy was 297,000 cells/mL in 2012 and 274,000 cells/mL in 2013. These values were 13 and 16% lower than the ASCCy in the respective years. In 2012, 82% of the farms shipped milk with a lower WSCCf than their ASCCf. In 2013, 97% of the farms shipped milk with a lower WSCCf than their ASCCf. The difference between a farm's WSCCf and its ASCCf tended to be greater in more-seasonal farms for BTSCC and milk volume. The WSCCm was lower than the ASCCm in every calendar month in both years. Collectively, these results show that the SCC of pooled milk from Florida was substantially lower than the arithmetic averages of monthly BTSCC values. Therefore, it should be made clear if the SCC is weighted by milk volume when average SCC results are reported. Programs to improve milk quality in Florida might be focused on conditions during August, September, and October because the BTSCC is then markedly increased on many farms.