Pregnancy rate is a key determinant of farm productivity and profitability
High pregnancy rates depend on a rapid return of normal reproductive function after calving. Both energy status and blood calcium levels impact ovarian and uterine function
Proceedings 2004 Florida Dairy Reproduction Road Show 10
The main objective of a dairy’s reproductive program should be to maximize pregnancy rate (PR) to first service. Pregnancy rate is the product of the heat detection and conception rate for the herd (PR = HDR x CR). Pregnancy rate represents the proportion of cows that become pregnant each estrous cycle, and determines the number of days that cows are open after the voluntary waiting period. When the PR increases because of a higher HDR, greater CR or both, days open decreases.
Ferguson and Galligan (13) have shown that PR to first insemination explained 79% of the variation in the calving interval. These authors concluded that maximizing the HDR and CR for first insemination is the most important determinant of calving interval. Therefore, dairy herds should allocate significant resources to maximize PR to first service.
To improve pregnancy rate to first service, estrous synchronization protocols such as the targeted breeding, modified targeted breeding or the OvSynch/timed AI programs are commonly used. By synchronizing a group of cows, estrus periods are concentrated within a 7-day period which helps to improve estrus detection rate or in the case of OvSynch/timed AI, cows can be inseminated without being detected in heat. These estrous synchronization or timed artificial insemination protocols increase PR because more cows are inseminated at the end of the voluntary waiting period.
However, to maximize first service PR from these protocols, dairy cows must have experienced multiple estrous cycles early postpartum. Cows expressing one or more estruses during the first 30 days postpartum had improved pregnancy rates to first service compared to cows with no estruses (41). The physiological and hormonal events associated with estrus are thought to help restore uterine and ovarian function to an optimal state for pregnancy establishment. The severity and duration of negative energy balance postpartum is a primary influence on ovarian activity and resumption of cyclicity postpartum in dairy cows (6,11).
Objectives of this presentation are 1) to describe the association between calcium status to periparturient disorders and its effect on postpartum energy status; 2) to relate the effect of postpartum energy status on reproductive performance; 3) to describe a protocol for management of the postpartum dairy cow with the ultimate aim of maximizing the pregnancy rate to first insemination.
Associations of Hypocalcemia to Calving Related Problems
During calving or shortly thereafter, hypocalcemia, characterized by blood calcium concentration < 8.0 mg/dl, is inevitable in the dairy cow (16,17,18,30,31). Hypocalcemia develops from the sudden drain of calcium to colostrum at the onset of lactation, resulting in a tremendous challenge to the cow’s ability to maintain normal calcium levels in blood.
Milk fever is the clinical manifestation of hypocalcemia and the decreased plasma calcium content is accentuated in affected cows. Affected cows are recumbent, are unable to rise, and have a calcium deficit of 8 grams. A standard intravenous dose of 500 ml of a 23 per cent calcium gluconate solution provides 10.8 gms of calcium.
Hypocalcemia may affect smooth muscle function in organs such as the uterus, rumen and the abomasum (stomach). A significant association between parturient hypocalcemia, dystocia and retained fetal membranes (RFM) in dairy cows, has been reported (7,8,19). Cows with parturient hypocalcemia were 6.5 times more likely to have dystocia, 3.2 times more likely to have RFM and 3.4 times more likely to have a left displaced abomasum (13). Evaluation of records from more than 61,000 dairy cows in Finland showed that parturient hypocalcemia was a significant risk factor for dystocia, RFM and clinical ketosis (19). Clinical ketosis was, in turn, associated with silent heats, cystic ovaries and infertility.
Hypocalcemia, without clinical symptoms of milk fever, may affect normal function of the uterus, rumen and abomasum. This condition is commonly called subclinical hypocalcemia and has been associated with various periparturient disorders (19,37). In a California study, hypocalcemia without paresis was more common in cows affected with uterine prolapse than in unafflicted cows (37). The prolapsed uterus was related to uterine atony, a delay in cervical involution and continued abdominal presses soon after parturition. Parturient hypocalcemia has been shown to delay cervical involution and cause uterine inertia (29). However, in primiparous cows, there was no difference in serum calcium concentrations between 9 cows which had prolapsed and their unaffected contemporaries (37). This supports the clinical finding that primiparous cows seldom experience milk fever.
Hypocalcemia has been associated with displaced abomasum and reduced rumen contractions. Cows treated for abomasal displacement had abnormally low blood calcium content preceding displacement (22). Cows with subclinical hypocalcemia were 4.8 times more likely to develop left displacement of the abomasum (26). Research with sheep demonstrated a true cause and effect relationship between hypocalcemia and normal smooth muscle contractility in the ruminant stomach (21). In this study, ruminal contractions ceased long before signs of hypocalcemia were observed. Moreover, ruminal dysfunction may occur substantially before the clinical signs of hypocalcemia. A field study comparing total serum calcium concentrations of cows diagnosed with abomasal displacement or volvulus to those of unaffected cows from the same herds, found hypocalcemia occurred in over two-thirds of cows affected with displaced abomasum or volvulus (10). This body of research suggests calcium administration at the time of treatment for displaced abomasum may be beneficial.
Postpartum dairy cows undergo a marked change in energy status as they are returning to normal restoration of ovarian cycles. Dairy cattle undergo a period of negative energy status in early lactation because energy output for milk production exceeds feed energy intake. Any calving related disorder that causes the cow to go off feed will aggravate the negative energy status already present during post partum.
Prolonged hypocalcemia after calving may suppress feed intake; cows with milk fever have been reported to have a lower dry matter intake post partum than non paretic cows (25). Further, hypocalcemia prevents secretion of insulin, preventing tissue uptake of glucose which would enhance lipid mobilization and increasing the risk for ketosis (24). Cows with milk fever had slower rates of uterine involution, attributed to a more severe negative energy balance and a greater loss of BCS rather than hypocalcemia directly (37). Hypocalcemia may result in the “droopy cow” syndrome sometimes observed early postpartum, even in cows that did not show clinical milk fever at calving. Goff et al (16), have reported that 10 to 50% of cows remain subclinically hypocalcemic (plasma calcium < 7.5 mg/dl) up to 10 days postpartum. Calcium treatment early post partum, particularly cows affected with dystocia or RFM, can help restore blood calcium concentration and promote normal function of calcium-dependent organs. Calcium treatment may smooth the transition from the dry period to early lactation.
Energy status of dairy cows during the first 2 weeks postpartum was found to have a large effect on integrated ovarian activity. Both cows cycling after 40 DIM and non-cycling cows were in progressively negative energy states, that is, they continued to be in a more negative energy state in the second week compared with the first week. This was especially true for the anestrous (non-cylcing) cows. Intake of feed by anestrous cows continually lagged behind that of cycling cows. Not only did anestrous cows eat less at week 1 postpartum, but intake differences increased as time went on. On the other hand, cows returning to CL activity the earliest started their recovery to a positive energy state immediately after the first week. The marked deficit in early energy status for the anestrous cows in this study exerted a marked carryover effect on conception. Only 33% of anestrous cows eventually conceived compared to 84% and 93% for early (before 40 DIM) and late cycling (40-63 DIM) cows, respectively.
Both the magnitude and severity of negative energy balance determine body condition scores (BCS) postpartum. Cows that lose more than 0.5 BCS, during postpartum have been reported to have compromised reproductive performance (11). Furthermore, pregnancy rates to first service are lower in cows with a BCS < 2.5 during the first 100 days postpartum (3,4,27,28). Because nearly all cows lose body condition postpartum, cows should be in good body condition at calving. A score of 3.25 to 3.75 at calving is recommended. Cows that are over-conditioned at calving are also candidates for excess body condition loss postpartum. Over-conditioned cows are unable to increase their dry matter intake quickly postpartum. As a result, body reserves are relied upon heavily to help support milk production. Over-conditioned cows were 2 weeks later in achieving a positive energy status than cows in good body condition fed high energy diets (20). One BCS unit (converted to U.S. system) was lost in order to support milk production by the overconditioned cows compared to a slight gain in body condition for control cows over a 10 week period (23). The reproductive problems of fat cows may not be due solely to lowered feed intakes. Fatter non-lactating cows have been shown to be less able to maintain a persistent follicle in the absence of a CL when exposed to progesterone from an intravaginal controlled internal drug releasing device (CIDR), whereas thinner cows maintained a persistent follicle (3). This difference in ovarian follicular response could be due to greater clearance of supplemental progesterone from the blood stream by fat cows.
Changing body condition through dietary manipulations requires some strategic planning and careful consideration. Under-conditioned cows should put on condition during the late lactation period because they are more efficient at utilizing metabolizable energy during this time than during the dry period (75 vs. 60%). In addition, the dry period may be too short to fully recover condition needed prior to calving. Cows should not lose weight during the dry period as the cow must gain 1 to 1.5 lb/day simply to meet the needs of the rapidly developing fetus.
The probability of conception to occur at first insemination can be determined by the loss of BCS during the postpartum period. A large study on the relationship between changes in BCS during the dry period, early lactation and conception to first service concluded that: 1) cows that lost one point of BCS in the 1st month of lactation were 1.5 times less likely to conceive than were cows that did not lose one point of BCS and 2) energy balance during the dry period and early lactation, as monitored by BCS, was more important to conception to first service than were health disorders or other risk factors evaluated (11). In several Florida field trials, body condition during the first 100 days postpartum was related to conception rate (3,5). An experiment was designed that compared pregnancy rates to timed insemination using the Ovsynch protocol for the first service of lactating dairy cows. At 9 weeks postpartum, cows were grouped into either low BCS (<2.5 BCS) or control groups (> 2.5 BCS) using a 1 to 5 scale (27,28). Pregnancy rates were less for the Low BCS group compared to the Control group at day 27 (18% vs. 34%) and at day 45 (11% vs. 26%). Rates of cumulative pregnancies through either 120 or 365 days postpartum were lower for Low BCS cows (P<0.01). A dynamic programing model can be used to demonstrate that additional revenue will be generated as the percentage of the herd with low body condition score (<2.5) decreases (42).
Relationship Between Elevated Crude Protein Intake and Energy Status
Changes in protein nutrition influences physiological changes associated with reproduction. If more crude protein (CP) is fed than can be utilized by the cow, urea concentrations in body tissues can be elevated. Feeding of diets containing 19 to 21% CP result in elevated BUN concentrations and frequently in lowered conception rates compared with cows fed 15 to 16% CP diets. Older cows are more likely to be affected negatively by elevated dietary CP than younger cows. Dietary concentration of degradable intake protein (DIP) is important, as well. Replacing soybean meal with a less ruminally degradable protein feedstuff such as fish meal, corn gluten meal, etc. often alleviates some reproductive inefficiency, including delayed first ovulation, lowered conception rates, and elevated embryonic deaths.
High protein feeding may reduce reproductive performance by increasing energy use for detoxification of ammonia, resulting in a "weakening" of the cow's energy state. The need to detoxify ammonia by animal tissues can be energetically costly. Feeding 100 g of unutilized CP will result in a loss of 0.2 Mcal of energy (43). If 500 to 1000 g of excess protein is consumed (2.2-4.4 percentage units of protein in the diet), energy costs could be substantial: 2 Mcal/d which equates to up to 7% of NEL requirement for maintenance and production of 30 kg of milk. With energy status averaging about -11 Mcal/d during the first three weeks postpartum (40), an additional 1 to 2 Mcal/d cost is not small. This energy cost is likely to push early postpartum cows even further into negative or less positive energy states, thus delaying return to normal ovarian activity.
Postpartum ovarian activity can be suppressed indirectly by feeding a high DIP diet (15.7% of DM), but this adverse effect can be alleviated partially by increasing dietary energy content (14,15). Pregnancy rate by 120 days postpartum was increased from 52.3% to 86.4% when dietary energy was increased with calcium salts of long chain fatty acids. The fact that elevated protein intakes may depress reproduction through increased energy costs to the animal is also supported by the work of Elrod and Butler (12). Feeding excess CP (21 vs. 15% of diet) lowered conception rates of heifers from 82 to 61% when heifers were fed an energy deficient diet (70% of ME requirements).
Successful management of lactating dairy cows needs to integrate the disciplines of reproduction and nutrition with standard postpartum herd health programs to optimize both milk and reproductive performance. In addition to milk fever, hypocalcemia appears to be a risk factor for dystocia, uterine prolapse, RFM and displaced abomasum, disorders which can negatively affect postpartum health and reproductive performance. Consequently, nutritional management strategies should be implemented during the last 3 to 4 weeks prepartum in order to promote a rapid return to normocalcemia early postpartum. The achievement of high energy intake, to bring cows out of a decreasing negative energy status as early as possible postpartum, is critical for both productivity responses. In the majority of lactating dairy cows, development of dominant follicles on the ovary occurs very early in the postpartum period. Low body condition scores at the time of insemination are associated with lower pregnancy rates to a detected or timed insemination. Feeding high levels of degradable protein results in greater loss of body weight and body condition which is associated with a decrease in ovarian activity. In contrast feeding of supplemental fat in the highly degradable protein diet can restore ovarian activity.
A time line protocol for the strategic management of postpartum dairy cows to ultimately maximize pregnancy rate to first insemination is outlined below. Consult your veterinarian on how to implement this protocol.
1. Transition cow nutrition:
Appropriate nutritional management of the prepartum transition dairy cow with the objective of reducing the incidence of hypocalcemia and energy related disorders (milk fever, dystocia, retained placenta, ketosis and metritis).
The following checklist is recommended to determine whether or not the nutritional management of the transition cows is appropriate to prevent these problems.
- The transition ration must be properly balanced for dietary-cation-anionic difference (DCAD), energy, fiber, vitamins and minerals content
- Are the cows eating 24 to 26 lbs of dry matter per day?
- Is there enough feedbunk space (at least 2 feet per cow)?
- Is there adequate shade in warm environments (50 square feet per cow)?
- Do you provide clean, well - designed calving facilities?
- Do you evaluate body condition score during the dry period?
- Do you determine urine pH and ketone bodies periodically to ascertain the DCAD and energy content of the ration?
2. Calving management:
Sound treatment and management of disorders associated with calving such as dystocia, milk fever, retained fetal membranes and udder edema. Who treats, what training have they received, when and how do they treat these problems?
3. Health monitoring of all postpartum cows during the first 10 days postpartum.
There are two general purposes for this program. First, to reduce the unnecessary use of antibiotics and hormones in cows that will not benefit from this type of treatments.
Second, it also assures that all postpartum cows are examined daily during the time when they are most susceptible to disease and most responsive to treatment. Health disorders such as, infection of the uterus, displacement of the abomasum and ketosis can be evaluated by monitoring rectal temperature, appetite, rumen function and urine ketones. At the time of disease diagnosis, cows should be treated promptly according to a farm protocol specified by the herd veterinarian.
4. Postpartum cow nutrition:
Is the postpartum transition cow ration properly balanced for energy, fiber, vitamins and minerals to maintain health and promote an early return to a positive energy balance? After calving, cows should be monitored for body condition, they should not lose more than one point of body condition score during the first 60 days after calving.
5. Breeding program at the end of the voluntary waiting period:
Application of the OvSynch timed artificial insemination protocol 60 to 80 days after calving to all cows.
This will assure that all cows receive an insemination at the end of the voluntary waiting period which results in an increase in the pregnancy rate to first service. Studies at the University of Florida have shown that timed insemination using OvSynch for all first service in both cool and hot seasons increased net revenue per cow by $16.57 36. After timed insemination, cows should be detected daily for estrus during the next 6 weeks and inseminated at detected estrus. Cows that have not been seen in estrus by the end of the 6 week period are palpated for pregnancy status. Cows that are found open can be re-assigned to the OvSynch/timed AI program.
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