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• An optimal TMR may be expected to result in improved production when initial injections of Posilac^TM are started at wk 9 of lactation, according to label.
• The time of highest daily milk production was around day 10 after injection, with a secondary peak on day 3.
• Abrupt decreases from peak milk production occurred in days 11 through 14, with second and older cows tending to reduce more than first lactation cows.

INTRODUCTION

Prior to the approval of Posilac^TM, many producers anticipated that initiation of rBST injections would be timed after the occurrence of some specific physiology thresholds, such as when cows were gaining body condition or weight, when midlactation had occurred, or after cows had become pregnant. However, the approval label specified that injections were to start at week 9 of lactation. While proven efficacy (an increase in milk production) to Posilac^TM was part of the FDA approval, the expected pattern of response was not known under some common management situations when injections were started at wk 9 of lactation. This trial was conducted to characterize some of the patterns that occurred when Posilac^TM was initiated at week 9 in cows of different breeds, ages, seasons, and to determine if milk production patterns were modified by feeding different TMRs that were designed to meet NRC requirements for the expected production during the experimental period.

MATERIALS AND METHODS

Ninety-one Ayrshires, Brown Swiss, Holsteins and Jerseys were used in the trial. Two TMRs were offered group-fed ad libitum. The AHigh@ TMR was formulated to meet the requirements of a 1400 lb cow, producing 88 lb of 3.5 percent FCM per day, and the ALow@ TMR was formulated to meet the requirements of a 1400 lb cow producing 66 lb of 3.5 percent FCM, according to NRC recommendations. The average chemical composition of the TMR=s are shown in Table 1. All cows received the High TMR starting at wk 7 of lactation (standardization time), and at week 9 the cows were assigned to one of the two TMRs according to calving date within breed and lactation number. At the end of the first period (after 18 weeks into lactation), half the cows in each group were switched to the other group. Posilac^TM was injected alternately at each side of the base of the tail at 14 days; thus cows received 5 injections (10 weeks) between wk 9 through wk 18 of lactation. Period II took place through weeks 19 to 26 of lactation and cows received 4 rounds of injections at 14-day intervals. In the analysis, cows were grouped into four seasons according to the calendar date of initiation of Posilac^TM treatments.

Table 1. Average Composition of the TMR=s fed to the High and Low Groups

RESULTS

Cows that received the "High" TMR produced significantly more milk and FCM than the cows that received the "Low" TMR in both periods. Actual milk yield production was higher in the first round of Period 1 than during wk 8 (before injection), and decreased slightly in each 14-day round. Fat and protein percentage increased during each round, resulting that FCM, fat production, and protein response was difficult to estimate accurately under specific conditions.

Results indicating patterns by day are summarized in Figures 1 through 2. Peak production was consistently achieved on day 10 in every round for both periods. A secondary peak occurred on day 3. A significant decrease in production occurred on days 13 and 14 in every round. After adjustment by covariance for production during wk 8, production was 0.73 lb/d higher for first lactation cows than multiparous in Period I and 2.73 lb/d in Period II (milk production in first lactation cows was slightly more persistently than older cows), although the differences were not significant at the 5 percent probability of significance. The differences between day 10 (high day) and day 14 (low day) were 2.51 lb/d for first lactation cows and 3.96 lb/d in second and older cows during Period I; for Period II, corresponding values were 2.46 and 2.99 lb/d. The production pattern clearly indicated that the effect of the Posilac^TM injection was greatly diminished by the time of the next date of injection that was scheduled at 14-day intervals.