By Joseph Dalton, Ricardo Chebel and J.L. Stevenson
CALDWELL, Idaho – Milk yield and reproductive performance of first lactation Holstein cows is closely related to age at first calving and weight after calving.
According to records processed at DHI Provo, the average age at first calving is greater than 25 months for Holstein herds in the western United States. In fact, less than 3% of U.S. dairy producers achieve the recommended target of ≤ 24 months of age at first calving and ≥ 1,230 lb (live weight) after calving. Increased age at first calving results in increased costs due to additional rearing expense, and lost income opportunity from not having milk in the tank earlier in the animal’s life.
The most common reproductive protocol for dairy heifers is insemination following detection of spontaneous heat. The proportion of heifers pregnant (formerly known as conception rate) following insemination after detection of heat usually ranges from 50 to 76%. Artificial insemination following detection of heat, however, requires daily observation for signs of heat and may result in an extended interval from puberty to pregnancy depending on heat detection efficiency and accuracy.
Other reproductive protocols used in heifers include the use of: 1) prostaglandin (PG), 2) an intravaginal progesterone insert (CIDR) and PG, and 3) timed AI (TAI). With good management, treatment of heifers with PG ensures greater heat detection and AI labor efficiency with no detriment to fertility when compared with heifers inseminated upon detection of spontaneous heat. The use of a CIDR and PG generally results in tighter heat synchrony compared with heifers synchronized with 1 injection of PG alone. Fertility of heifers synchronized with a CIDR and PG (and receiving AI after heat detection) is comparable to heifers that receive AI after PG-induced or spontaneous heat.
Unfortunately, heifers inseminated at a fixed time after the completion of Ovsynch have reduced fertility compared with those inseminated after heat detection. The disappointing results following TAI in heifers may be related to the increased number of follicular waves in heifers as compared to cows, leading to difficulty in achieving ovulation following the first GnRH injection of Ovsynch. In lactating cows, for example, it is clear that the proportion of cows that become pregnant after Ovsynch is dependent on the proportion of cows that ovulate in response to the first GnRH injection.
Recent research at the University of Idaho investigated four different reproductive protocols at a large Idaho dairy. Holstein heifers approximately 13 months old were assigned to one of four protocols (Figure 1). Heifers in the control group received daily tail paint and AI on detection of spontaneous heat. Heifers in the PG group received one injection of PG at enrollment and AI on detected heat, whereas those not inseminated received a second PG injection 14 days later and AI on detected heat. Heifers in the CIDR group received a CIDR insert for 7 days, a PG injection at CIDR removal, and AI on detection of heat during 3 days after CIDR removal. Heifers not inseminated by 72 hours after CIDR removal received TAI + GnRH injection. The last group was the TAI group and the protocol was as follows: GnRH injection, 6 days later insertion of CIDR and injections of PG and GnRH, 7 days later CIDR removal and PG injection, and TAI + GnRH injection 48 hours after CIDR removal.
The first GnRH injection was intended to presynchronize heifers so that maximum ovulation to the GnRH given 6 days later would be achieved. The PG injection given at the time of CIDR insertion was intended to cause regression of CL, reduce the progesterone concentration, and improve synchronization of ovulation at the time of TAI.
Results are shown in Table 1. Proportion of heifers pregnant 32 ± 3 days after AI was greater for control and PG heifers compared with CIDR and TAI heifers. The interval from enrollment to pregnancy among heifers that became pregnant was also affected by treatment (Table 1). Heifers in the PG and CIDR groups had the shortest interval, whereas TAI heifers had the longest. The 28-day pregnancy rate (the proportion of heifers pregnant at the end of the 28-day breeding program, as determined by the formula: number pregnant/total number of eligible heifers) was smallest for the heifers in the TAI group as compared to heifers in the PG and control groups.
Data used for evaluation of economic outcomes were based on actual data from the dairy. The reproductive protocol costs were calculated based on costs of drugs, labor costs for treatment of animals, and labor costs for heat detection. The rearing cost was calculated based on daily maintenance cost per heifer multiplied by the number of days from enrollment in the study to pregnancy or to the end of the study period for heifers that did not become pregnant. The breeding program costs (Table 2) were calculated by adding the reproductive protocol cost and the rearing cost. Breeding program cost was lowest for control and PG groups, whereas heifers in the CIDR group had an intermediate breeding program cost, and TAI heifers had the highest breeding program cost (Table 2).
The mean cost per pregnancy generated (Table 2) for each treatment was also calculated. (More detailed information on the economic analyses can be found at: Stevenson et al., 2008. J. Dairy Sci. 91:3124-3438). Heifers in the PG and control treatments had the lowest cost per pregnancy followed by heifers in the CIDR and TAI treatments, respectively (Table 2).
The TAI protocol used in this study resulted in an extended interval from enrollment to AI and conception, lower pregnancy per AI, and a smaller proportion of heifers pregnant at the end of the 28-day breeding program. Consequently, heifers in the TAI group had a greater cost per pregnancy generated. In contrast, the control and PG protocols, which were based on detection of spontaneous or PG-induced heat and subsequent AI, resulted in reduced reproductive protocol costs and rearing costs, respectively. Consequently, heifers in the control and PG groups had the lowest breeding program cost and cost per pregnancy generated. Ultimately, treatment of heifers with PG every 14 days until insemination and pregnancy resulted in the best economic outcomes.
Efficient and accurate heat detection is necessary for successful reproductive management in non-TAI protocols. Consequently, in a situation where AI technicians struggle with heat detection efficiency and accuracy, the control and PG protocols could result in decreased pregnancy rates, which could affect the economic outcomes. Nevertheless, the results shown here provide evidence that simple reproductive protocols, based on detection of spontaneous or PG-induced heat, can result in high reproductive efficiency and be economically viable.
■ To contact Dr. Joseph Dalton, e-mail him at: email@example.com.