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Larger Herds Frequently Yield Lower Somatic Cell Counts in Florida

11 February 2015

Scale of production, as well as scale of seasonality, have been two interesting variables when looking at somatic cell counts in milk tanks on Florida dairies.

Florida summers are tough on cows - milk production drops and cows are more susceptible to mastitis.

This is down to heat stress and is reflected in higher somatic cell counts (SCC), according to two University of Florida experts.

In an article appearing in the University's Dairy Update, Professor Albert DeVries and PhD student Fernanda Ferreira demonstrate how average SCC rises and milk production declines in the later summer in Florida. These seasonal trends cannot all be attributed to the direct effects of heat stress they write. 

We know that calving is somewhat seasonal on most farms with most of the calving happening in the fall and early winter, writes Professor DeVries.

By the time the summer arrives, more cows are in the later stages of lactation and more cows are dry. Cows later in lactation generally produce less milk and milk with a higher SCC. Calving patterns therefore explains part of the drop in milk production and rise in SCC in the summer.

The opposite effects of a lower milk volume and higher SCC made us wonder what the average SCC of all pooled milk in Florida is (more about that in a later article). We also wondered if there was an association between herd size and SCC.

We asked Southeast Milk, Inc., and Premier Milk, Inc., both milk marketing cooperatives, for the monthly volumes of milk shipped and average monthly bulk tank SCC of their Florida members in 2013. Both cooperatives sent us the data with the herd identifications removed. After removing some incomplete records, we had data from 100 farms in 2013, or about 77 per cent of all licensed dairy herds in Florida.

In Figure 2 we show the relationship between annual amount of milk shipped and the annual average SCC for the 100 Florida farms. Not all farms are shown in figure 2 to prevent identification through milk volume.

For each farm, the average SCC in figure 2 is the arithmetic average of the 12 monthly bulk tank SCC values that we received from the cooperatives. Figure 2 shows that farms that shipped lower volumes of milk tended to have a higher annual bulk tank SCC.

The average SCC for all farms in figure 2 was 327,000 cells/ml. For the 79 farms that shipped less than 25 million pounds annually, the average SCC was 345,000 cells/ml. The average SCC for the 21 larger farms was 261,000 cells/ml. It is also clear from figure 2 that there are many smaller farms with lower SCC. Fourteen farms had an annual average SCC < 200,000 cells/ml and 79 farms had an annual average SCC < 400,000 cells/ml.

We were also interested in the seasonality of the monthly bulk tank SCC data from the 100 Florida farms. The data showed that February, March, and April were generally the months with the lowest bulk tank SCC. We called this the cool period.

August, September and October were generally the three months with the highest bulk tank SCC. We called these months the warm period. A measure of seasonality is the warm to cool ratio. We calculated the warm to cool ratio as the average SCC in the warm period divided by the average SCC in the cool period for each farm.

For example, if the ratio = 2, then the average SCC in the warm period is twice as high as the average SCC in the cool period. Figure 3 shows the seasonality of the bulk tank SCC for the 100 farms. We see that all but two farms had a warm to cool ratio > 1. That means that on 98 farms, the SCC in the warm period was greater than in the cool period. On 53 farms was the warm to cool ratio < 1.5. These farms had an average SCC of 312,000 cells/ml. The other 47 farms had an average SCC of 344,000 cells/ml.

The data in figure 2 and 3 show that many Florida farms produced milk with a bulk tank SCC that would look good anywhere in the US. Their milk quality management practices might be studied and could be implemented on other farms.

For some farms, producing milk with a low SCC seemed less of a priority. We also see an increase in the SCC in the summer months for almost all farms. To calculate the average SCC in all pooled milk on individual farms we need to know how seasonal their milk production is.

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