In the current study a lactation curve described by a low starting milk yield and a steep slope was associated with early conception across all parities. This concurs with an earlier investigation which found that high early milk yield was associated with a longer period of days open . The present study indicate that this difference is observable before 15 DIM (Figure 2). A too high starting milk yield would presumably maintain the NEB and thereby prolong the time to conception . An increased number of A.I. per conception indicate impaired reproduction (Table 1). The material used, however, could not answer if delayed breeding start is voluntary or a result of silent heat.
The peak milk yield seemed to have no association with the time of conception. Higher milk yield has been associated with longer time to resumption of ovarian activity in high yielding dairy cows in some reports [8, 19, 20]. Other authors have found no [21, 22] or inverse associations between milk yield and recurrence of ovarian activity [23, 24].
In the current study a steep ascending slope early in lactation is associated with early resumption of breeding activity. The biological reason for this might be the found in energy coverage. Earlier studies have found that the change in body condition score and the magnitude of the negative energy balance (NEB) in early lactation are crucial to conception [25, 26]. A steep ascending slope of the lactation curve may be indicative of adequate energy coverage during the early post partum period and thereby early resumption of ovarian activity, found in the current study. An earlier Norwegian study found that cows experiencing early versus delayed resumption of ovarian activity have lactation curves with different slopes as energy balance approaches zero throughout the post partum period . A risk of choosing a feeding strategy based on a high escalation rate of concentrates to ensure adequate energy coverage is the increased risk of indigestion because of too sudden changes in the ruminal environment. Cows of the current study that succeeded in becoming pregnant appeared to have counteracted this negative effect of concentrate as indigestion is likely to lead to lower milk yield and impaired reproduction [28, 29].
A confounding variable not taken into account in the current study, because of the lack of data, is the voluntary waiting period (VWP), which is the time period decided by the farmer before breeding starts. Although the average 305 d yield in the current study was less than 7000 kg, one cannot rule out that a high milk yield early in lactation, might have lead farmers to prolong the VWP to avoid the problems related to drying of in some high yielding cows. On the other hand, a high yielding cow might be looked upon by the farmer as hard to get pregnant and therefore started to be bred earlier i.e. a shorter VWP, which is supported by a high number of A.I in late conceiving cows (Table 1). In other words, management decisions based on high yield early in lactation, consistent with a steep ascending slope, might influence the calving to conception time in either way. However, the principal aim of this study was to investigate whether there is an association between the lactation curve and time to conception in Norwegian dairy cows. The reason for delayed conception may be biological, or managerial, or as is more often the case, a combination of both. Nevertheless, the alteration in the profile of the lactation curve may pose an opportunity of early identification of a herd reproductive problem that reaches beyond estrus detection and A.I. management. When the problems are to be ruled out at the herd level by trained personnel, additional information on voluntary wait, culling policy and feeding management will probably be readily obtainable from the herdsmen.
The current study identified an association between a steep descending slope of the lactation curve and early conception. The reason behind this association might be the negative effect pregnancy has on milk yield [30, 31]. This effect and its magnitude on milk yield is beyond the scope of this paper and we hope to be able to pursue this question in future research programs.
In the current study the lactation curves were estimated by using a modified Wilmink model and the statistical setup of a mixed model using monthly test day records of daily milk yield. A concern with the current method may be that milk records obtained after conception might influence the estimated shape of the curve before conception. The fit of the curves generated from using all milk records to raw data proved to be satisfying (Figure 1). Potential confounding of correlation between test-day milk yields and clustering within lactation was taken care of by running the model with repeated measurements of test day milk yield within lactation nested within herd.
Another concern related to the association between conception time and the shape of the lactation curve is culling of animals because of reproductive failure. The farmer's decision whether to give up on getting a cow pregnant may be influenced by the course of the lactation curve. Cows with a less persistent lactation curve, i.e. a steep descending slope of the lactation curve, may be given up earlier than other cows. This would cause a problem if the primary aim of the study was to assess reproductive performance at herd level. The following reasoning applies to the selection criteria used in the current study where non-pregnant cows and culled cows, 30.2% of the observations, were excluded. The most prevalent reasons for culling are low milk yield, reproductive failure, disease and age . Cows omitted from the study for non-pregnancy are likely to have followed the lactation curve pattern of late conceivers with a high persistency of the lactation curve [30, 31]. Including these cows would probably have emphasized the difference in lactation curve traits between late and early conceiving cows. There might be a risk however, that the late conceivers are overrepresented among high performing cows. The reason for this being the increased likelihood of culling a low performing cow compared to a high performing cow. In the material used in the current study, peak milk yield was virtually equal across calving to conception intervals within each parity category (Table 2), and therefore this bias of performance is believed to be minimal.
Disease is known to have an effect on both milk production  and reproductive performance . To avoid the influence of disease on the association, only data from lactations with no records of veterinary treatments (68.3%) were used in the current study. The data used in the current study were obtained from the NDHRS database, which has not been validated yet. A major and currently ongoing Nordic research project will validate all the Nordic dairy health recording systems. There might be problems with non-directional misclassification and measurement errors which weaken the statistical power of the result using non-validated sources. Research has shown, however, that adjusting the sample size can account for this potential loss of power  and this is well taken care of by the large sample size used in the current study.
The strength of the current study, using a large representative database and a large sample size, is that the results can be implemented on the study population of Norwegian dairy cattle. One must consider, however, that we have restricted our analysis to certain calving season July to September. The reason for this approach is that internal validity is more important than external validity. We also, however, tested this for different calving seasons and found the same association. Therefore we are confident that an association between the time of conception and the shape of the lactation curve does indeed exist. Plotting daily milk records obtained from the automatic milking system, the shape of the lactation curve might be monitored, adjusted by feeding and optimized for conception. Together with heat detection and insemination technique this is yet another tool to manage the reproductive performance in the dairy herd.