Introduction Risk factors for poor reproductive performance

Reproductive performance of dairy cows influenced a herd’s profitability, and good heat detection and conception rates provided opportunities for management control. It was not always economically advantageous to get cows pregnant as soon as possible, and there was no one optimal value for the calving interval length for all cows in a herd. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Dairy cows; Reproduction; Epidemiology

1. Introduction

Profitable milk production relies upon a careful, efficient and cost-effective manage- ment of dairy herds. Poor reproductive performance is an important production-limiting factor. Management decisions on dairy farms should be economically justified. Our ultimate goal is to help farmers make the best decisions regarding management of their dairy herds. To do this, we must understand the biological parameters involved, namely milk yield, disease, and reproductive performance. Although many studies on reproductive performance in dairy cows have been conducted, few have integrated together epidemiology and economics. Many of them have been constrained by the lack of three factors: a large health database, computing power, and sophisticated statistical tools. Fortunately, we have had access to high quality Ž Ž . data on Finnish Ayrshires beginning with Grohn et al. 1986 , and continuing until the ¨ . present ; such data have not been previously available in the United States, where Holsteins predominate. Therefore, we established a health and production database in Ž Ž . the Northeast USA beginning with Grohn et al. 1995 , and continuing until the ¨ . present to study the epidemiology and economics of production diseases under North- eastern management. In this presentation, our focus is reproductive performance in dairy cows. We will Ž . Ž . concentrate on three aspects of it: 1 risk factors for poor reproductive performance; 2 Ž . consequences of poor reproductive performance; and 3 economic optimization of reproductive performance. The material for this review was primarily taken from our on-going epidemiological research on New York Holsteins and Finnish Ayrshires.

2. Risk factors for poor reproductive performance

2.1. Increased risk of reproductiÕe disorders The dairy industry’s major goal is to provide milk for the consumer market. Over the past several decades, milk yield of cows has increased markedly. However, one negative Ž impact of this improvement is reduced fertility e.g., cows open for longer, more . services per conception and higher incidence of reproductive problems, which, in turn, Ž Ž .. contribute to reduced fertility e.g., Dhaliwal et al. 1996 . Some have argued that higher disease incidence and reduced fertility in high-producing herds result more from poor management than from biology. High milk yield may contribute to a negative energy balance in some cows, especially those still growing, and disease may occur. Selective culling muddies the issue further. High yielders are more likely to remain in the herd, and receive more veterinary treatment, even when they become ill, than low yielders. To address this question, we conducted several studies in both Finnish Ayrshires and Ž . New York Holsteins e.g., Grohn et al., 1990, 1994, 1995; Rajala and Grohn, 1998a . ¨ ¨ Ž Lactational incidence risks for reproductive disorders in our latest Finnish study Rajala . and Grohn, 1998a were: dystocia, 2.1; retained placenta, 3.1; metritis, 3.2; silent ¨ heat, 8.1; ovarian cysts, 7.3; and other infertility, 1.9. In our New York study Ž . Grohn et al., 1995 , incidences were higher: retained placenta, 7.4; metritis, 7.6; ¨ and ovarian cysts, 9.1. Using logistic regression, we analyzed the effect of several risk factors on the Ž . occurrence of reproductive disorders. In one study Grohn et al., 1990 , first parity cows ¨ Ž . were most likely to have dystocia Table 1 . Older cows were more likely to have retained placenta and ovarian cysts. Winter calvers had higher risks of early metritis, silent heat, ovarian cysts, and other infertility than summer calvers did. Most reproduc- tive disorders were interrelated. For example, dystocia was a risk factor for retained Table 1 Ž . Risk factors parity, calving season, diseases for seven diseases, in 61,124 Finnish Ayrshires. All seven models included community as a proxy of management a Risk factors Diseases DystociaRetained placentaOvarian CystsEarly metritisLate metritisSilent heatOther infertility Parity U 1 2.1 – 1.0 – – – – U U U 2 1.0 1.0 1.2 – – 1.6 1.7 U U U 3q 1.3 1.5 1.4 – – 1.1 1.1 CalÕing season U U Jan–Apr – – 1.0 1.6 1.0 1.9 1.3 U U May–Aug – – 1.0 1.0 1.6 1.9 1.3 U U U Sept–Dec – – 1.6 1.7 1.9 1.0 1.0 Diseases U U Dystocia – 4.1 – 3.2 – – – U U U U Abortion – 19.8 – 3.7 3.5 2.1 – U U U Retained placenta– – – 4.4 2.5 1.3 – U U U U Early metritis – – 1.5 – 1.6 1.5 1.7 U U U Late metritis – – 2.9 – – 3.0 2.4 U Silent heat – – 1.5 – – – – U Prolapsed vagina – – 3.3 – – – – U U U U Ovarian cyst – – – 5.4 2.2 1.6 2.6 U U Other infertility – – 2.8 – 3.6 – – a Values are odds ratios, which measure how much more or less likely the outcome is among observations Ž . Ž with a given risk factor or level of risk factor if 2 categories , compared with those without it or reference . category, which has an odds ratio of 1.0 . Milk yield was also included in these models. U p- 0.05. placenta and metritis. Results were similar in our study on New York Holsteins, for Ž . parity and calving season, but there were fewer disease associations Grohn et al., 1995 . ¨ Cows with higher milk yield in the previous lactation were more likely to have retained placenta, early metritis, silent heat, ovarian cysts, and other infertility than were cows with lower milk yield. Cows with higher milk yield in the current lactation were more likely to have ovarian cysts than were cows with lower milk yield. In a similar Ž . study on New York Holsteins Grohn et al., 1995 , higher milk yield was associated ¨ only with ovarian cysts. In addition to breed difference, other factors may account for the greater number of disorders associated with milk yield in Ayrshires than Holsteins. Finnish farms are much smaller than New York farms so management differs. The Finnish studies contained over five times as many cows as our New York study, so smaller differences become significant. Also, in New York, a farmer may treat animals for disease, but in Finland only veterinarians treat animals. 2.2. Risk factors for delayed conception Many factors influence reproductive performance in dairy cows, only some of which are under the farmer’s control. Milk yield may play an important role in reproduction. However, the relationship between milk yield and conception is difficult to study because of the confounding effect of culling. Conception does not necessarily occur at the first breeding; cows may have to be inseminated several times. A cow that remains Ž . open i.e., does not conceive for a long time or has low milk yield is more likely to be culled. Some researchers have argued that high yielders are less fertile than low yielders are; this association may, however, be complicated by selective culling. The objective of our New York study was to measure the effect of 60-day cumulative Ž . milk yield on rates of conception and first breeding in lactation Eicker et al., 1996 . Data were from 15,320 Holsteins calving between June 1990 and November 1993, in 26 herds. We used survival analysis, which allows inclusion of all cows, whether or not they had conceived or been bred by the end of the study. Thus, the loss of information was minimized. Parity, calving season, and herd as a proxy of management were included as confounders. Ž Retained placenta, metritis, and ovarian cysts were risk factors for conception Table . 2 . Cows with retained placenta had a 14, those with metritis a 15, and those with ovarian cysts a 21 lower conception rate than cows free of these disorders. Current cumulative 60-day milk yield had no effect on conception rates, except among the Ž . highest yielders. They had a slightly lower but not significant conception rate than did the lowest yielders. Because these results may be a consequence of farmers intentionally delaying insemination of high yielding cows, we also studied the association between milk yield and time to first insemination. As 60-day milk yield increased, so did insemination rates. The highest yielders were nearly 30 more likely to be inseminated than were the lowest yielders. Our results on New York Holsteins indicate that milk yield in the first 60 days has only a minimal effect on conception. Only the highest yielders had a slightly lower Table 2 Effect of milk yield, parity, calving season, and disease on conception in 13,307 New York Holsteins a Risk factor Hazard ratio First 60-day cumulatiÕe milk yield kg F1582 1.0 1583–1891 0.99 1892–2195 1.01 2196–2541 1.01 2541 0.92 Parity 1 1.0 2 0.98 UU G 3 0.92 CalÕing season Dec–Feb 1.0 U Mar–May 0.93 Jun–Aug 1.06 Sep–Nov 1.01 Disease UU Retained Placenta 0.86 UU Metritis 0.85 UU Ovarian Cysts 0.79 a Hazard ratios for factors in proportional hazards model. The hazard ratio is the ratio between two relative Ž . Ž . risks of an event e.g., conception . For example, if a cow has a hazard ratio of 0.92 parity G 3 , then she has an 8 reduced likelihood of conceiving than a first parity cow. U p- 0.05. UU p- 0.01. conception rate than their herdmates. Older cows and sick cows were less likely to conceive. In contrast, the rate of being bred increased with 60-day milk. These findings demonstrate that farmers are making rational decisions by breeding young, healthy, high yielding cows. Among 30,036 multiparous Finnish Ayrshires, the lowest yielders were less likely to Ž . conceive than were average yielders Harman et al.,1996a . Among 11,761 heifers, the highest yielders were less likely to conceive than were average yielders. A number of disorders decreased conception probability in both multiparous and primiparous cows: anestrus, ovulatory dysfunction, other infertility, late metritis, and clinical ketosis Ž . Harman et al., 1996b .

3. Consequences of poor reproductive performance