Major changes are occurring in the reproduction of high producing lactating dairy cows.
Reproductive management strategies to deal with these changes are discussed.
Changes in some reproductive measures in lactating dairy cows
Time to first ovulation
First ovulation is often delayed in lactating dairy cows with time to first ovulation averaging 33.3 ± 2.1 days in Holstein cows in the U.S.A. (compilation of 10 studies reported in Ferguson, 1996). This delay is generally attributed to the period of negative energy balance during the early postpartum period in dairy cattle, and a reduction in the pulsatile luteinizing hormone (LH) secretion needed to stimulate the final stages of follicle growth and estradiol production ( Savio et al., 1990; Staples et al., 1990; Beam and Butler, 1997; Lamming and Darwash, 1998; Roche et al., 2000; Butler, 2001).
Although delayed first ovulation is associated with negative energy balance, it is not as clearly associated with level of milk production.
Negative energy balance and inadequate follicular growth can explain a portion of anovulatory dairy cows but does not seem to be an adequate explanation for every case. Similarly, a simple relationship between anovulation and level of milk production does not appear to exist. More complex physiological models are needed to fully explain anovulation in dairy cows (Gumen and Wiltbank, 2002; Wiltbank et al., 2002).
Fertility is a complex trait that is likely to be related to numerous factors, including uterine infection, negative energy balance, urea concentrations in the blood, vitamins, fertility of sire, accuracy of estrous detection, insemination technique, etc. (Faust et al., 1988; Staples et al., 1990; Ferguson, 1996; Lamming and Darwash, 1998; Gröhn and Rajala-Schultz, 2000; Roche et al., 2000; Royal et al., 2000; Butler, 2001; Lucy,
2001; Moreira et al., 2001; Gong et al., 2002; Washburn et al., 2002; Lopez-Gatius, 2003; Santos et al., 2004a; Santos et al., 2004b). For example, increased double ovulation rate in high- producing dairy cows increases the chance of pregnancy, even though high milk production is expected to decrease the odds of an ovulated oocyte to produce a pregnancy. Thus, a simple relationship between milk production and conception rate seems unlikely.
Duration of estrus
Low rates of estrous detection are reducing reproductive efficiency on commercial dairy farms. Estrous detection rates in southeastern U.S. Holstein dairy herds have decreased from 50.9% in 1985 to 41.5% in 1999 (Washburn et al., 2002). However, studies have reported both negative relationships between level of milk production (Harrison et al., 1989; Harrison et al., 1990) and no relationship (Fonseca et al., 1983; Van Eerdenburg et al., 2002) using twice daily visual heat detection.
Duration of estrus in a group of lactating dairy cows was evaluated using continuous monitoring of all mounts (HeatWatch system, Lopez et al., 2004). Cows with milk production above the herd average (~ 88 lb/day) had shorter (P < 0.001) duration of estrus (6.2 ± 0.5 h) than cows with lower milk production (10.9 ± 0.7 h). This effect was not due to parity; primiparous and multiparous cows showed similar responses.
High producing cows (103 lb/day) had larger follicles (18.6 ± 0.3 versus 17.4 ± 0.2 mm diameter; P < 0.01) but lower circulating estradiol (6.8 ± 0.5 versus 8.6 ± 0.5 pg/ml; P < 0.01) compared to lower producing cows (71 ± 1.3 lb/day) (Lopez et al., 2004). Duration of estrus increased as peak estradiol concentrations rose (positive correlation, r = 0.57; P < 0.0001), but decreased as milk production increased (negative correlation, r = -0.51; P < 0.0001). Higher levels of milk production were associated with smaller follicular size (negative correlation, r = -0.45; P < 0.0001).High milk production was postulated to lead to decreased circulating estradiol concentrations, resulting in decreased duration of estrus.
Double ovulation rate
Double ovulation rate has been directly linked to milk production (reviewed by Wiltbank et al., 2000; Lopez et al., 2005a). Double ovulation rate appears to be the underlying cause of increased twinning rate in lactating dairy cows, with 93% of twins being nonidentical (Silvia Del Rio et al., 2004). Numerous factors have been recognized as possible regulators of twinning rates, including age of dam, season, genetics, use of reproductive hormones or antibiotics, ovarian cysts, days open, and peak milk production [reviewed in Wiltbank et al., 2000]. In a large study on risk factors for twinning, Kinsel et al. (1998) concluded, “the single largest contributor (> 50%) to the recent increase in the rate of twinning is the increase in peak milk production”. Other publications show similar relationships between milk production and double ovulation rate (Fricke and Wiltbank, 1999; Lopez et al., 2005a).
A dramatic inflection point in double ovulation rate has been shown as milk production increases above 88 lb/day; it is unclear what physiological changes above this critical value. The level of production within the 2 weeks prior to ovulation is the most influential (Lopez et al., 2005a). As with duration of estrus, the first postpartum ovulation differed from other ovulations, showing a high double ovulation rate that was unrelated to milk production (Lopez et al., 2005a).
Circulating steroids and steroid metabolism in lactating dairy cows
As discussed above, cows with higher milk production ovulate larger follicles but have lower circulating estradiol (Lopez et al., 2004) and progesterone concentrations (Lopez et al., 2005a). Other studies have reported similar changes in circulating hormones and size of ovarian structures in lactating cows (Ahmad et al., 1995; Inbar et al., 2001).
There appear to be two reasonable explanations for the disconnection between circulating steroid hormones and size of follicles and corpus luteum (CL). The first possible explanation is that follicles and CL are less steroidogenically active in lactating dairy cows. This could be due to insufficient levels of circulating stimulatory hormones, substrate for steroidogenesis, or intracellular steroidogenic pathways. The hypothesis that ovarian structures in lactating dairy cows have reduced steroidogenic output has not been investigated adequately to support or refute this hypothesis. The second, more likely explanation is that steroid hormone metabolism increases as milk production increases.
Circulating hormone concentrations are determined by rates of production and metabolism of the hormone. Increased feed consumption, such as during lactation, has been shown to alter circulating progesterone and excretion of progesterone (Parr et al., 1993a; Parr et al., 1993b; Rabiee et al., 2001a; Rabiee et al., 2001b). Liver blood flow and metabolism of progesterone and estrogen increased immediately after any amount of feed consumption in both lactating and non-lactating cows (Sangsritavong et al., 2002). Further, metabolism of estrogen and progesterone was much greater (2.3 X) in lactating than in non-lactating cows (Sangsritavong, 2002; Sangsritavong et al., 2002). The changes in estrogen and progesterone metabolism after feeding are immediate and appear to be related to acute changes in liver blood flow. In lactating cows, a continuous high plane of nutrition appears to chronically elevate liver blood flow and metabolism of steroid hormones to approximately double the amount observed in similar size and age non-lactating cows. These results indicate that even with a similar level of hormone production, there would be lower circulating hormone concentrations in lactating dairy cows.
Although we cannot rule out the importance of changes in steroidogenic production by luteal and follicular tissue, it seems reasonable that the changes in circulating estradiol and progesterone can be accounted for by increased rates of steroid metabolism in lactating cows. We have synthesized this information into a simplified working model. Lactating cows have greater energy requirements than non-lactating cows. The high feed consumption required to meet these requirements leads to a dramatic increase in liver blood flow (Sangsritavong, 2002; Sangsritavong et al., 2002) which leads to elevated metabolism of both estrogen and progesterone with a subsequent reduction in their circulating concentrations, despite high production of steroid hormones by the follicle or CL. Therefore, it makes sense that a higher producing cow would have a shorter duration of estrus because of increased steroid metabolism. Thus, this model provides a logical and likely explanation for the changes in duration of estrus, and for the paradox of lower circulating steroids but larger ovarian structures occurring in lactating dairy cows.
In addition, the model explains how elevated steroid metabolism due to high milk production could reduce fertility. The preovulatory follicle and oocyte would be exposed to a longer period of elevated LH pulses that could lead to ovulation of an overstimulated oocyte and reduced fertility (Ahmad et al., 1995; Ahmad et al., 1996; Revah and Butler, 1996). A reduced rate of progesterone rise following ovulation could also reduce fertility, as has been suggested by others (Folman et al., 1973; Ahmad et al., 1996; Dunne et al., 1999; Mann, 2001).
However, this model does not yet explain how very high milk production (> 88 lb/day) can produce dramatic increases in double ovulation rate. Our recent intensive study of hormonal changes associated with selection of single, double, or triple dominant follicles in lactating dairy cows demonstrates that reduced circulating estradiol near follicle selection is not responsible for multiple dominant follicles (Lopez et al., 2005b), as we originally proposed (Wiltbank et al., 2000). Nevertheless, circulating progesterone is decreased and LH and FSH are increased near the time of selection, making it possible that changes in hormonal metabolism may still have a role in this process. Future manipulation of reproductive function in lactating dairy cows will require clearer information on the precise effects of elevated steroid metabolism on reproductive physiology in lactating dairy cows.
Practical reproductive management implications
The next section will briefly suggest some practical implications and reproductive management strategies for problem areas.
Decreased duration of estrus due to high milk production
What does this mean on a commercial dairy farm? We used the data on duration of estrus versus milk production to estimate what would happen to heat detection efficiency for cows with different levels of milk production. If a cow is producing about 70 lb/day, a 4-time per day heat detection program will detect about 90% of cows that are in estrus. However, this same program (4 times/day) will only detect about 50% of cows in heat if they are producing above 100 lb/day. The results are even worse if heat detection is done only twice per day or once per day. It should be noted that all of the probabilities in this analysis were based on actual ovulation by the cows (detected by ultrasound). Some producers will say that high producing cows are not cycling; however, they are cycling normally. Instead, these cows are not detected in heat because of extremely short heat duration.
Increasing frequency of heat detection can help solve this problem. Many heat detection aids, such as tail chalk, to help find cows that are in showing heat when observers are not present. This can be critical because high producing cows are showing heat for 4 hours or less in many cases. Most dairy producers in the United States are incorporating timed AI programs, such as Ovsynch, into their reproductive management programs to get highproducing cows bred in a timely manner.
Treating anovular cows
Although level of milk production is not normally associated with incidence of anovulation, dairy producers still need to treat anovular cows. Generally, 20% of dairy cows will not be cycling by 70 days after calving. This percentage will increase with a high percentage of low BCS (2.5 or less) cows. Anovular cows need to be quickly assigned to a hormonal program (and possibly nutritional program if they have low BCS) that will start the cows cycling. An Ovsynch program alone is not the ideal treatment for anovular dairy cows. Use of a CIDR® (Pfizer, Inc., New York, NY) or estradiol should be incorporated into these programs for optimal results.
Increasing double ovulation rate (and twinning rate) with increasing milk production
From a practical standpoint, there may be little that we can do to change this trend. It seems clear that the main increase occurs after cows are producing about 90 lb/day. Thus, we must anticipate a dramatic increase in double ovulation rate in high producing cows which will result in an increased twinning rate for the cows that conceive. Management procedures take into account this increasing twinning rate if milk production is increasing. First, twin pregnancies must be diagnosed. Second, to management procedures should be adapted for cows that are likely to have twin births. Twinning cows will calve earlier (10 to14 days on average) and are likely to have more problems during the calving process. Cows at risk for twins are the highest producing cows during the previous lactation.
Decreasing conception rate due to higher milk production
As discussed above, many factors impact conception rate in lactating dairy cows, including higher milk production. On many farms, production level may be a fairly minor factor. The effect of milk production on fertility is dramatically amplified during hot weather. Increased body temperature during heat stress leads to decreased reproductive success, particularly early embryonic death.
Since reduced conception rates seem to stem from events during the first week after insemination, transferring a good quality embryo at 7 days after expected time of AI should improve reproduction. In a fairly large experiment, conception rate in the University of Wisconsin herd was compared for cows were bred by AI with those undergoing embryo transfer (ET). However, ET did not improve overall conception in lactating cows, suggesting other timeframes are also important in conception failures.
Many laboratories are experimenting with alterations to timed AI programs that may increase conception rates in high producing dairy cows. There are numerous intriguing possibilities, but data is insufficient to change recommendations at this time.
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