PRINT page | RETURN to previous page

Cows Given bST Remain in Herds,
Stay as Healthy as Non-supplemented Cows

Michael Howie, Feedstuffs Staff Editor
April 12, 1999

A study by two Cornell University researchers that examined eight years of data on the effects of bovine somatotropin (bST) has indicated that cows stay as healthy and remain in herds as long as cows in non-supplemented herds.

The researchers, in a paper in the proceedings from the 1999 Cornell University Cooperative Extension Winter Dairy Management meetings, said scientific studies have given "remarkably consistent" results, showing that bST enhances milk yield and increases productive efficiency. The goal of their current study was to look at bST response under field conditions, paying particular attention to response to bST over the lactation cycle and response over the four years since approval.

Researchers on the project were D.E. Bauman and R.W. Evertt of the department of animal science at Cornell University, and R.J. Collier of Monsanto Co.

In order to pull together necessary information for the study, Dairy Herd Improvement (DHI) records were utilized from Northeast DHI, Vermont DHI Assn. and Pennsylvania DHI Assn. and were matched at the university with a list of bST users and nonusers supplied by Monsanto. bST users were defined as producers who used bST on or before June 1994 and had continuous use through March 1998 and supplemented at least 50% of the eligible cows in milk.

From all this data, the researchers identified 340 herds with a total of more than 27,000 cows in milk. Of these, 176 were control herds (non-bST users) and 164 were bST users. They said only Holstein cows were included in the analysis. Table 1 shows the average of production parameters on test day for control and bST herds. The researchers said the parameters in Table 1 are the average of more than 2 million cow test-day records analyzed for the 340 herds.

They said control herds were slightly smaller than bST herds and were lower in production before the use of bST ("pre" - defined as January 1990 through February 1994) and after bST was available ("post" - defined as July 1994 through March 1998). Data available to the researchers included milk, fat, protein, somatic cell count linear score (SCC), age in days on test day and days in milk (DIM) on test day.

The researchers analyzed the data using the test-day model (TDM) for each herd for both the pre and post periods. TDM is designed to account for systematic biological and environmental changes that occur in a set of data, they said.

The researchers said two sets of solutions, pre-bST and post-bST, were available from each herd for analysis of management on test day, age, DIM in the first lactation, DIM in the second lactation, month fresh and days carried calf. They said the pre and post solutions were compared to determine changes in traits caused by bST, assuming all other changes were the same in the two groups of herds. "While there were undoubtedly some changes within individual herds in both groups, this would tend to balance out with the large number of herds," they said.

The experimental design defines the response to bST as the difference pre and post in the control and bST herds. The researchers said these differences can be estimated from management on test day, the shape of the lactation curves or any other component of TDM.

The researchers said any change in herd production due to weather, feed supply, education, milk price, etc., should be similar for both groups or should change at the same rate in both herds. Because of this, they said any additional differences would "logically be associated with bST supplementation."

Table 2 shows the changes in management milk related to management improvements in the control herds over the period studied. The researchers said TDM adjusts test-day production such that management milk represents production as if the cows were the same age, stage of pregnancy, stage of lactation and freshened all in the same month on all test days. Therefore, the researchers said, Table 2 shows the improvement in produciton achieved without the use of bST. Overall, from 1990 to 1998, daily production per cow increased 6.97 lb. of milk while SCC linear score decreased 0.208 units.

Table 3 shows the changes in bST herds for the same period of time as in the control herds in Table 2. The researchers said changes in Table 3 represent the result of bST plus the normal management improvements that caused changes in Table 2. Therefore, they said, results in Table 3 minus results in Table 2 should show the response due to bST. These results are shown in Table 4.

The researchers said the results presented in Table 4 are the response in production due to bST supplementation and represent the average per cow that is milking on test day. "Thus, the data represent an average for all milking cows in the herd with the assumption being all cows in the herd are supplemented on every test day," they said, adding that in reality, only a percentage of the herd received supplemental bST and producers do not treat every eligible cow with bST. They said 1993 was set to zero in Tables 2-4 to provide a comparison of the changes that occurred pre- and post-bST.

The researchers also plotted lactation curves to estimate the response of bST. They said the data clearly showed no difference between the control milk production first lactation curves for the two time periods studied. "This indicates that management changes in the two time periods in Table 2 did not change the lactation curves in the control herds," said the researchers.

However, bST use changed the shape of the curve. The researchers said response to supplementation appeared to increase with increased days in milk. The response peaks at 8 lb. of milk per cow per day and declines in late lactation -- although the researchers said this assumes that all the first lactation cows were supplemented with bST, which was not true, so this response should be considered the "minimum."

For cows in their second or later lactation, the researchers said the spread was the same -- with 8 lb. of milk response being gained by supplementing bST. In fact, the researchers said if it was assumed that 75% of cows were supplemented following day 60 of lactation, the response would peak at nearly an additional 11 lb. of milk per cow per day.

As for the effect of bST on SCC, the researchers plotted lactation curves for SCC and compared pre- and post-bST periods.

The researchers found that SCC varies over the lactation cycle, being high in early lactation, decreasing during the first 100 days postpartum and then progressively increasing over the rest of the cycle. This was a similar pattern for both the control and bST periods. The pattern was also similar for cows in their second or later lactation.

The researchers concluded bST had no impact on and was of no consequence to average milk SCC, did not affect average age in the herd and did not affect average days in milk.

PRINT page | RETURN to previous page

HOME | CONTACT