Introduction
The profitable operation of a dairy herd is influenced by the reproductive performance of the lactating cows. The level of reproductive performance that optimizes profit is a result of the combination of the days that a cow spends in the most efficient time of the lactation curve and the cull rate due to reproductive failure.1 The main factors that determine reproductive performance in dairy herds are the voluntary waiting period (VWP), insemination rate, pregnancy per artificial insemination (AI), and pregnancy loss.2 The outcome of these factors depends on the reproductive herd health program of the herd.3
Typically, reproductive programs on dairy herds are focused on hormonal manipulation of the estrous cycle to obtain a pregnancy in a timely manner at the end of the VWP. However, due to the relationship of biological and management factors that determine when cows become pregnant at the end of the VWP and whether or not pregnancy is maintained, a reproductive program needs to integrate the principles of theriogenology and herd health in order to optimize reproductive performance.
This chapter evaluates the importance of the outcome of the factors that affect reproductive performance considering a herd health approach and discusses management strategies to improve herd fertility.
Impact of postpartum diseases on the VWP
The VWP is the set time postpartum that producers choose not to breed cows to allow the uterus to recover from infection and for cows to achieve a positive energy balance and resume estrous cyclicity. These conditions can be viewed as physiological requirements for an optimal time to pregnancy at the end of the VWP. The challenge for the dairy cow is that following parturition they experience some degree of hypocalcemia, and due to increased nutrient demands for milk yield early postpartum, a state of negative energy balance (NEB) occurs, which affect uterine health and estrous cyclity.
Hypocalcemia and NEB appear to prevent cows from mounting an effective immune response to the microbial challenge to the uterus after calving. Ribeiro et al.4 reported that low calcium concentration in the first 7 days postpartum was associated with an increased incidence of metritis. Martinez et al.5 reported that dairy cows that developed metritis had decreased calcium concentrations (< 8.59 mg/dL) in the first 3 days postpartum, and was associated with decreased neutrophil function. Further, the relative risk of developing metritis decreased by 22% for every 1 mg/dL increase in serum calcium in the first 3 days postpartum.5 In a study by Hammon et al.,6 cows that were in NEB because of low dry matter intake before and after calving had suppressed neutrophil function and developed subsequent uterine infections postpartum. Dubuc et al.7 reported that risk factors for metritis among dystocia and retained fetal membrane included increased nonesterified free fatty acids (≥ 0.6 mmol/L), a marker for NEB in the first week postpartum. Energy status has been linked with a delayed resumption of estrous cyclicity and ovulation postpartum.8 NEB reduces the frequency of luteinizing hormone (LH) pulses, thereby impairing follicle maturation, and inhibits estrous expression in part due to a reduction of estrogen receptor α in the brain.9
There has been much discourse in the literature regarding the association between high milk yield and NEB in lowering fertility in dairy cows over the past 30 years. Nonetheless, little or no association has been observed between milk production in early lactation and the risk of anovulation, pregnancy, and pregnancy loss in high-producing dairy cows.10 However, of greater concern to producers is the high incidence of health disorders postpartum, particularly those that affect the reproductive tract and those of metabolic origin, on reproductive performance. Santos et al.11 evaluated data from 5719 postpartum dairy cows daily for health disorders. All cows were evaluated for cyclicity at 65 days postpartum and were subjected to presynchronized timed AI programs to their first service. Only 55% of cows were considered healthy and did not develop clinical disease in the first 60 days postpartum. Incidence of diseases observed were as follows: calving-related problems, 14.6%; metritis, 16.1%; clinical endometritis, 20.8%; fever, 21.0%; mastitis, 12.2%; ketosis, 10.4%; lameness, 6.8%; digestive problems, 2.8%; and pneumonia, 2.0%. Twenty seven per cent of the cows were diagnosed with a single disease event, whereas 17.2% had at least two disease events in the first 60 days postpartum. Although these health disorders did not influence milk yield (10 919 kg, 11 041 kg, and 10 858 kg for cows considered healthy, those with a single disease, and those with multiple diseases, respectively), they depressed cyclicity, reduced pregnancy at the end of the VWP, and increased risk for pregnancy loss. Moreover, those cows that experienced two or more health disorders had lower pregnancies to first AI and higher pregnancy loss than cows with one disorder. In contrast, healthy cows achieved a high (51.4%) pregnancy per AI at the end of the WVP and had lower odds for pregnancy loss.
These studies indicate that improving postpartum health by preventing periparturient diseases has the potential to improve fertility by increasing pregnancy per AI and lowering pregnancy loss. That is, cows that transition well from parturition to lactation are more likely to have good reproductive performance in their lactation. Below is a checklist that veterinarians can follow to determine whether or not a transition cow program is amenable to lowering the incidence of periparturient disorders.
- Is feed intake maximized?
- Is there feed available at all times?
- What is the frequency of feed delivery?
- What is the number of cow movements during the prepartum and postpartum transition period?
- What is the feed bunk space (60–76 cm/cow)?
- Are diets formulated to prevent postparturient diseases?
- What is the fiber content?
- Prepartum cows should consume daily about 15–18 Mcal of NEL and 1.1 kg of metabolizable protein.
- What is the mineral composition of the diet to avoid hypocalcemia?
- What is the level of employee competency in providing calving assistance?
- What is the incidence of hypocalcemia and ketosis? Has it changed over time?
- Is there a postpartum health program for prompt diagnosis and treatment of sick cows (metritis, ketosis, mastitis)?
What is the ideal VWP?
The ideal VWP for an individual herd is partly determined by the pregnancy rate (PR = number of cows pregnant/number of cows eligible to breed in a 21-day cycle) of the herd. Ribeiro et al.12 modeled the desired VWP for a herd according to the ideal day postpartum at pregnancy and the PR of the herd. Two calculations were considered, one for an ideal median day at pregnancy of 110 and another of 130 days postpartum. Herds with poor PR (<13%), even if they begin to breed immediately after calving, cannot achieve any of those days to pregnancy. In those herds with pregnancy rates above 15%, these days for pregnancy can be achieved but the VWP has to be around 30 days, which is not ideal. In contrast, those herds with excellent PR (>22.5%), days to pregnancy of 110 and 130 days can be achieved with a VWP of 60–98 days. A survey by the National Animal Health Monitoring System13 reported that the average VWP is 55 days postpartum regardless of herd size. This range in days for the VWP is likely related to the typical PR of 16–17% found in dairy herds in the United States.12
Parity, milk production, and persistency are cow factors that should also be considered when establishing the length of the VWP. De Vries14 reported on the optimal VWP and days to pregnancy as a function of these factors in Holstein herds in the United States. The optimal VWP or first insemination for the average first-parity cow is 77 days and 70 days for the average second- and third-parity cow. Low-producing cows could start their breeding period sooner and first insemination for higher-producing cows could be delayed by 1–2 weeks.
Strategies to increase pregnancy rate at the end of the VWP and decrease the interval between inseminations
The use of prostaglandin (PG)F2α promotes estral events that are concentrated within a 7-day period, which helps to improve estrous detection. However, whether or not cows are inseminated depends on the efficiency of estrous detection, which is low (<50%) in most herds.12 Consequently, it is now well accepted that an alternative for increasing insemination rate and increasing the proportion of pregnant cows at the end of the VWP is the incorporation of timed AI programs.15 Timed AI programs are particularly beneficial on farms with low detection of estrus, particularly in herds with 21-day cycle insemination rates below 55%.2 Application of timed AI increased the yearly profit by $30 per cow compared with estrous detection alone.16 Similarly, $80–148 lower cost per pregnancy for timed AI compared with estrous detection programs was reported by De Vries.14 Timed AI programs were considered to improve reproductive performance because AI submission rates increase; however, increased knowledge of the reproductive biology of the cow and how hormonal manipulation influences oocyte and embryo quality have now led to the development of methods that also optimize pregnancy per AI.4 Indeed, pregnancy rates per AI above 45% have been achieved with timed AI programs in high-producing cows and 55% in grazing dairy cows.17,18 Within an AI program, those herds that incorporate timed AI for first insemination followed by detection of estrus experience the lowest median days to pregnancy and more profit per cow.4 In a simulation of reproductive performance and economics of various breeding programs by Galvão et al.,19 the highest 21-day cycle PR was obtained when cows were subjected to timed AI for first AI, with 95% compliance of treatments followed by efficiency of estrous detection at 60% with 95% accuracy. This program also resulted in the shortest median days to pregnancy and increased profit from increased milk production. Therefore, incorporation of timed AI with high compliance to first service, followed by an efficient estrous detection program, improves pregnancy rate and increases profit. Further, in order to optimize reproductive performance dairy herds cannot abandon estrous detection practices altogether.
Use of bulls in a natural service (NS) breeding program is an option that some dairy producers use to avoid the need for detection of estrus and to improve PR. A study conducted in the southeast region of the United States which compared NS with AI after detected estrus demonstrated that reproductive performance was not different for NS herds.20 In contrast, Overton and Sischo21 (using data from 10 large California dairy herds that utilized a combination of AI and NS) reported that cows were more likely to become pregnant if they were bred by AI. Further, in a direct comparison between timed AI and NS, two breeding systems without estrous detection showed similar reproductive performance.22 However, NS was more expensive ($10.00–60.00 per cow annually) than timed AI, related to the cost of feeding bulls, milk price, genetic merit, and whether or not to replace bulls in the lactating pen by cows.23
When to identify nonpregnant cows?
