Determination of farm factors related to reproductive efficiency

Reproductive efficiency is declining on U.S. dairy herds. There are numerous factors that affect reproductive efficiency on individual farms. These include heat detection rate, heat stress, semen quality, bull fertility, AI technique, nutrition, body condition score, clinical and subclinical mastitis, cow comfort, and many other factors related to whole farm management, individual farm managers and laborers, and individual cows. This manuscript will briefly summarize 2 recent research areas. The first section of the manuscript discusses a recent study on body condition score. This study was part of a larger study that evaluated numerous factors simultaneously to determine the management factors most related to reproductive performance (Caraviello et al., 2006). This study used a machine learning technique to simultaneously evaluate numerous factors (341 different variables) that may be affecting reproductive efficiency on U.S. dairies. Interestingly, one of the top factors that was found to be closely associated with reproductive performance was the amount of bunk-space per cow in the breeding pen. The predicted percentage of cows that were pregnant by 150 days tended to increase linearly as bunk space increased from 30 to 60 cm (from 35% to 70%). Thus, cows that were overcrowded (<36 cm of bunk space/cow) had a much lower percentage of cows that were pregnant at 150 days in milk than cows with more bunk space.

Another factor that was closely related to reproductive efficiency was body condition score. This has been identified as a key factor in numerous previous studies. We used reproductive data from 153 large commercial dairies that participate in the Alta Advantage progeny-testing program. After editing, the data set had 17,587 lactation records from 9,516 cows that calved from 2000 to 2004 on 153 large (average of 613±46 lactating cows), commercial dairies across the United States. We obtained BCS on a subset of these cows and did an initial analysis to find the average values of BCS for cows at different stages of lactation. This was done to determine the normal BCS in these herds and to set a "normal" value for all stages of lactation that could be used to determine the "BCS status" of each herd. Mean BCS for dry cows was 3.22; this declined to 2.88 by 30 d postpartum and declined further to 2.70 by 50 d postpartum. Mean BCS subsequently increased to 2.75 by 80 d postpartum and stabilized at around 2.85 by 200 d postpartum. The herds in this study ranged from nearly 0% to more than 60% BCS faults. The relationship between percentage of BCS faults and reproductive performance was evaluated at the herd level by comparing herd BCS status with the percentage of cows pregnant by 150 d postpartum. Pregnancy status at 150 d postpartum was chosen as the dependent variable, because, as discussed above, it is a robust measure of overall herd reproductive performance. Application of a logistic regression model at the herd level showed that the proportion of cows pregnant by 150 d postpartum decreased linearly as the percentage of BCS faults increased. The probability of pregnancy by 150 d postpartum was 0.80 for herds with 15% BCS faults, as compared with 0.53 for herds with >45% BCS faults. Cow level evaluation of the relationship between BCS faults and probability of pregnancy by 150 d postpartum confirmed trends that were observed at the herd level. Individual cows with BCS faults had significantly (P < 0.05) lower probability of becoming pregnant by 150 d postpartum (0.61) than cows without faults (0.74).

Thus, we have developed a simple system to separate herds according to BCS status determined at a single farm visit. This BCS "fault" system can be used to identify herds in which BCS problems may be the cause of poor overall reproductive performance. Alternatively, herds can be identified in which BCS faults should not be viewed as a likely cause of poor reproductive performance and therefore other areas should be examined.

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High feed consumption and reproductive efficiency are linked by high hormone metabolism.

This second section of the manuscript will not attempt to review all of the nutritional factors that can potentially alter reproductive efficiency in dairy cattle. This has been reviewed in numerous other publications and would require a complete volume for adequate review. We will focus on a critical link that has become increasingly relevant to our understanding of nutrition and reproduction. This link is very different from the link discussed above concerning BCS and reproduction. Negative energy balance and the resulting low BCS cause a delay in the time to first ovulation and thus increase the percentage of cows that are anovular (not ovulating). The increased percentage of anovular cows is not clearly tied to milk production in most studies. However, there are other key reproductive efficiency measures that are clearly related to milk production. This review will focus on 2 key reproductive measures, efficiency of heat detection and double ovulation rate. These are chosen because of the clear relationship to milk production. We will use these measures to build a model for how reproductive efficiency can be dramatically altered by the high feed consumption associated with high milk production. These 2 measures are used to clearly demonstrate the physiological model that can explain the relationship between high feed consumption and decreased reproductive efficiency. However, this model becomes much more complicated when trying to explain a much more complex reproductive variable such as conception rate. Nevertheless, it seems likely that this model will also be important in explaining some aspects of this variable as well.

We theorize that high milk production leads to decreased circulating estradiol concentrations producing decreased duration of estrus. Decreased estradiol could also cause increased follicular size by delaying the time to estradiol-induction of estrus, GnRH/LH surge, and ovulation in high-producing cows. Alternatively, high milk production causes increasing double ovulation rate. This would lead to a much greater twinning rate in lactating dairy cows. The mechanisms and practical

Authors

Milo C. Whitbank
Kent A. Weigel
Daniel Z. Caraviello
Department of Dairy Science
University of Wisconsin-Madison