Reproductive management of dairy heifers
Raising replacement heifers is a substantial cost of dairy operations. Some authors estimate it to be at approximately 20% of the total cost, and there is no immediate return from the heifer enterprise either. Therefore, many dairymen try to minimize the costs associated with heifer rearing. Although this is a perfectly sound strategy, when “minimizing costs” becomes neglecting heifer rearing, profitability of dairy herds is jeopardized. A significant aspect of proper heifer rearing is good reproductive management, without which a longer interval to onset of productive life will occur.
Holstein heifers achieve puberty at 40 to 50% of their expected adult body weight (520 to 700 pounds). Puberty is defined as the occurrence of ovulation followed by the formation of a corpus luteum (CL) and a luteal phase of normal duration that would elicit the establishment of a pregnancy. During the estrous cycle, which lasts approximately 22 days, most heifers have two (56%) or three (33%) follicular waves, with a few (11%) having four. This is somewhat different than lactating cows that have predominantly two follicular waves (Sartori et al., 2004). The fact that nearly 50% of heifers have three or more follicular waves poses a significant challenge for the development of timed AI protocols for heifers. Further, it is widely accepted that heifers have significantly better estrous expression and P/AI (percentage of heifers pregnant after AI) than lactating cows, which reduces the need for timed AI protocols.
The most important factor affecting estrus-detection rate is weight of heifers upon initiation of the breeding program. In an Idaho study, heifers weighing 745 pounds or less were significantly less likely to be inseminated within 11 (60.5 vs. 70%) and 22 (95.4 vs. 98.8%) days after the initiation of the breeding program than heifers weighing greater than 745 pounds. This is expected considering that only pre-pubertal and pubertal heifers display estrus, and onset of puberty is dependent on weight. Importantly, however, heifers of appropriate weight at the beginning of the breeding program had very good estrous detection rates, which is expected when heifers are synchronized with prostaglandin (PG) F2α. In one of our studies, we compared estrus expression of heifers treated and not treated with PGF2α at the beginning of the breeding program (Figure 1; Stevenson et al., 2008). Among heifers treated with PGF2α, 63% displayed estrus between day 2 and 4 after the beginning of the breeding program and 72% displayed estrus within a week. Among heifers not treated with PGF2α, 11% displayed estrus between day 2 and 4 after the beginning of the breeding program and only 36% displayed estrus within the first week. In these herds, heifers were restrained once a day by self-locking head stanchions and detection of estrus was based on removal of tail paint that was applied daily. This is an important finding because it indicates that as long as heifers are of appropriate size and weight, they will demonstrate strong signs of estrus and estrous detection rates are very high.
The P/AI of heifers inseminated in estrus is also expected to be significantly greater than the P/AI of lactating cows. Heifers that had their estrous cycles synchronized with PGF2α injections and were inseminated based on signs of estrus had P/AI of 67.8 and 65.5% at 40 and 90 days after first AI (Chebel et al., 2007). Other researchers have obtained similar P/AI in heifers inseminated based on signs of spontaneous or PGF2α-induced estrus P/AI (67.8% – Stevenson et al., 2008; 60% – Lopes et al., 2009). Therefore, one can expect that well managed heifers inseminated in estrus should have P/AI greater than 60% after first AI. The most important factors affecting P/AI in heifers seem to be service sire, AI technician, exposure to extreme temperatures (cold and heat stress), and immunization (Donovan et al., 2003; Chebel et al., 2007).
In general, submission of heifers to timed AI protocols results in significant smaller P/AI than insemination based on signs of estrus. Table 1 compares P/AI of heifers inseminated with different timed AI protocols or based on signs of estrus. Recently, however, a new timed AI protocol based on intra-vaginal progesterone insert (CIDR insert for 5 days, PGF2α injection at CIDR removal, and AI+GnRH 72 hours after CIDR removal) has resulted in good P/AI (approximately 58%). The cost of this timed AI protocol, however, is approximately $12.15 per heifer, whereas the cost of a PGF2α+estrus detection synchronization protocol is approximately $5 per heifer (Stevenson et al., 2008).
Reproductive management of heifers has to start with raising them properly so that heifers of appropriate weight and size are inseminated. Considering that heifers express strong signs of estrus and heifers inseminated in estrus have very good P/AI, if labor and facilities are appropriate, dairy herds should implement PGF2α+estrus-detection synchronization protocols.
|Table 1. Pregnancy per AI (P/AI) and pregnancy rates (PR) of heifers submitted to timed AI or estrus-synchronization protocols.|
Estrus Detection + AI
|Protocol||Heifers||PR||Reference code*||Protocol||Heifers||P/AI||PR (21-d) %||Reference code*|
|6-d Cosynch 48h||175||34.3||4||No synchronization||135||63.7||56.0||7|
|6-d Cosynch 48h||95||29.5||5||PGF2α-induced estrus||127||67.7||60.9||7|
|6-dCosynch 48h+CIDR||94||31.9||5||PGF2α-induced estrus||141||60.0||45.4||8|
|6-d Cosynch 48h||82||45.1||6||Overall||6,972||65.2||66.9|
1-Schmitt et al. (1996); 2-Pursley et al. (1997); 3-Stevenson et al. (2000); 4-Rivera et al. (2004); 5-Rivera et al. (2005); 6-Rivera et al. (2006); 7-Stevenson et al. (2008); 8-Lopes et al. (2009); 9-Rabaglino et al. (2010); 10-Lucy et al. (2001); 11-Chebel et al. (2007).
Published in Dairy Star October 1, 2010