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Daily Milk Fluctuations in BST Injected Cows
by Gene C. McCoy and Michael F. Hutjens

The use of BST(bovine somatotropin) to stimulate milk production by the dairy producer has gained increasing acceptance since its availability in February, 1994. Considerable amount of research has been reported showing increased milk production when BST was injected at 14 day intervals in lactating Holstein cows. The purpose of this report to illustrate the effects of BST on lactating Ayrshire and Jersey cows, daily variation when BST is injected at 14 day intervals, and heat stress responses.


Seven Ayrshire and eleven Jersey lactating cows were injected with BST (Posilac) at 14 day intervals for ninety-six days starting with the first injection on May 9,1994 (500 mg sometribove zinc/injection). During the month of June, high ambient temperatures occurred. All cows received a TMR ration comprised of alfalfa haylage, corn silage, and a grain mix balanced according to NRC for 60 pounds of milk and fed ad libitum. Daily milk weights and weekly body weights were recorded.


Results of the experiment are illustrated in Figure 1. Ayrshire cows (averaged 183 days in milk or DIM) increased after the first injection of BST (5-9) from 47 pounds to 61 pounds while the Jersey cows (182 DIM) increased from 43 pounds 52 pounds and continued increasing until after the third injection of BST( 6-6) at which time high environmental temperatures occurred causing heat stress and decreased milk production. No control cows were available to compared to the BST-injected cows. Figure 1 illustrates the cyclic nature in milk production after each injection of BST. Daily milk production increased during days 1-7 after the injection of BST and decreased during days 8-14.

Figure 2 illustrates Jersey daily milk production after the cows were divided in two production levels (cow over 45 and cows under 45 pounds per day) prior to the start of injections of BST. Jersey cows in the high production group averaged 130 DIM while the low production group averaged 225 DIM. Similar patterns were observed as illustrated in Figure 1. It appears the low production group recovered from the heat stress and had greater milk persistency than the higher group throughout the trial. All cows gained approximately .86 pound per day during the trial.

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