A calving interval of 1 year must be maintained for optimal beef breeder production. Several factors including degree of calf suckling, reproductive failure, genetic variation, and level of maternal nutrient intake are known to affect postpartum reproductive efficacy of beef cows [12,15,16]. Among these, one of the major factors contributing to prolonged calving due to conception interval is inadequate nutrition, which often causes subclinical and/or clinical metabolic diseases [5,12]. Prepartal and postpartal nutritional statuses are also related to the interval between calving and first ovulation in beef cattle [12]. Thus, it is assumed that certain metabolic parameters that might reflect the protein and energy status of cows during prepartum and postpartum would be associated with reproductive performance of breeding cattle herds.

We recently reported a Japanese Black breeding herd with extremely low blood urea nitrogen (BUN) concentration (mean, 2.2 mg/dL; range, 1.6~4.0 mg/dL), presumably due to the nutritional quality of fed roughage harvested from the home pastureland. This herd had no clinical symptoms (including body condition scoring); however, both BUN concentration and reproductive performance of the herd were considerably improved after dietary supplementation with crude protein [17]. Our follow-up study reconfirmed the usefulness of our metabolic profile test (MPT) parameters, particularly BUN and glucose (Glu), not only for determining the metabolic nutritional status of the breeding cattle herds in our region, but also for selecting the dietary remedy for each herd to improve reproductive performance. Additionally, our findings identified regional differences in the MPT results, which reflected the inadequate nutritional content of the roughage, possibly arising from regional pastureland differences [18]. Our previous results suggested that MPT parameters such as BUN and Glu may be suitable for evaluation of the metabolic nutritional status of Japanese Black breeding cattle, which may greatly affect reproductive performance, at least within our region.

In the present study, retrospective surveillance was conducted to confirm the relationship between metabolic parameters collected pre- and postpartum and postpartum reproductive performance based on our MPT database generated from Japanese Black breeding herds in our region. Furthermore, we evaluated the relationship between metabolic parameters and seasonal conditions, which might vary according to seasonal changes in dietary roughage and heat stress during the summer period.

Animals were cared for according to the Guide for the Care and Use of Laboratory Animals (Joint Faculty of Veterinary Medicine, Kagoshima University).

The main objectives of this retrospective surveillance of our database were to investigate and clarify the blood metabolic parameters that might be associated with reproductive performance and reflect seasonal differences in Japanese Black breeding cattle herds in our region. Our results suggest that protein (TP and Alb) and energy (FFA and Glu) metabolisms, as well as hepatic cytotoxicity status, affect reproductive performance of breeding cattle in our region during perinatal periods. Additionally, results revealed that metabolic statuses of protein and energy of breeding cattle might be affected by seasonal differences, which could manifest as differences in dietary harvested grasses, particularly in our region, or reduced feed intake due to heat stress during summer time.

A significant tendency in TP (p = 0.054) with no differences in Alb, A/G ratio, and BUN were observed between the good and poor groups in evaluation 1, suggesting increased globulin levels in the poor group. Globulins comprise a heterogeneous group of proteins that includes antibodies and other inflammatory molecules, as well as hemostatic and fibrinolytic proteins [1]. Therefore, it was assumed that certain inflammatory factors within the body during the prepartum period, with no accompanying clinical symptoms or increased protein intake, might affect postpartum reproductive performance. The data describing the postpartum period revealed a significant difference in GGT activity. GGT is a membrane-bound enzyme found in cells with high rates of secretion or absorption. Although GGT activity is present in many tissues, it is primarily considered a serum marker for diseases of the hepatobiliary system associated with cholestasis [6]. Thus, although the cause of increased GGT in our study herds was not determined, Fasciola hepatica infection was common in cattle in our region, indicating that it is a possible cause, and might have affected reproductive performance.

Interestingly, when we compared each parameter between pre- and postpartum periods in the good and poor reproductive performance groups, significant differences were observed in TP (increase), Alb (increase), AST (increase), Glu (increase), and FFA (decrease) in the good group, but not the poor group. It has been suggested that metabolism of protein plays an important role in the regulation of physiological functions during the perinatal period [2,6]. Additionally, the activity of aminotransferases in blood is very important for the metabolism of amino acids and carbohydrates; thus, the increased aminotransferase activity in the blood (reflected as increased AST in our study) might have been a consequence of increased activity within normal reference values in the cells or cell structural damage, as previously reported [2,6]. Therefore, the results of these comparisons between good and poor groups may indicate essential patterns in our established MPT system for achieving improved reproductive performance in the breeding cattle herds in our region. Accordingly, further study with an expanded MPT database is warranted.

Unsatisfactory reproductive performance has been acknowledged by most farmers after artificial inseminations during periods in which cattle were fed barnyard grass silage (digitaria) as dietary roughage in our region, suggesting that these long-term feeding customs of the farmers may be affecting reproductive performance of the herds in our region. Therefore, we conducted a preliminary evaluation to clarify the effects of seasonal change in dietary roughage from Italian ryegrass to barnyard grass silage and possible heat stress during summer on MPT values in our region. Significant differences in both TP and A/G ratio were observed between groups during both pre- and postpartum, implying that these differences must be derived from differences in globulin concentration. As mentioned above, globulins are a heterogeneous group of proteins that include antibodies and other inflammatory molecules, as well as hemostatic and fibrinolytic proteins [1]; thus, although the underlying cause of the difference in globulin concentration was not evident from our results, the above findings, together with the results of evaluation 1, indicate that the occurrence of inflammation without clinical symptoms may affect reproductive performance in breeding cattle herds during summer in our region.

In the present study, significant differences in Glu between seasonal groups were observed in both the pre- and postpartum periods. It has been suggested that blood Glu has more potential as an indicator of carbohydrate status and dietary starch content than as an indicator of energy balance, illustrating that, at the same degree of negative energy balance, cows fed with a higher proportion of grain had higher blood Glu concentrations than those receiving a lower proportion of grain [3,4]. Conversely, Katamoto et al. [7] previously examined the influence of seasonal differences in VE concentration in Japanese Black breeding cattle and reported significant decreases in VE during summer, which might have been related to decreased feed intake due to high ambient temperature. Additionally, Matsumoto et al. [11] reported that subclinical health status might have affected consumption and resultant VE concentration. In the present study, no significant differences were observed in VA and VE concentrations between summer and non-summer periods in samples collected during the pre- and postpartum periods. This lack of seasonal effect might have been caused by vitamin supplementation of the breeding cows by farmers in our region throughout the study period. The difference in Glu in the present study might have reflected differing energy and/or carbohydrate status of the roughage fed between seasons, or reduced feed intake due to high ambient temperature. Thus, Glu may be a useful parameter for monitoring the energy status of breeding cattle herds that do not show seasonal differences in vitamin concentrations or clinical symptoms during summer.

Pre- and postpartum nutritional levels have been reported to be the most important variables for reducing the interval between parturition and first ovulation [5,14]. Additionally, BUN level has been shown to be a sensitive indicator of the balance between available digestible crude protein and energy fed to a ruminant [8], which can facilitate measurement of the efficiency of protein utilization [9,10]. We recently confirmed these findings by revealing improved reproductive performance in Japanese Black breeding cattle herds through the use of dietary remedial measures based on evaluations of MPT parameters, particularly BUN and Glu, in each breeding herd in our region [17,18]. The results suggested a possible relationship between protein/energy balance, reflected as BUN and Glu levels, and reproductive efficacy in each breeding cattle herd within the MPT database in our region. Interestingly, in the present study, no relationships emerged between BUN concentration and reproductive performance (evaluated by the number of open days after calving) in samples collected in the pre- and postpartum periods. The reason for these results is not clear. It has been reported that BUN concentration reflects current dietary protein intake, and concentration of Alb is affected by long-term protein status because of its long half-life [3,10]. Therefore, possible reasons for our study findings are that all databases were evaluated without considering regional differences in BUN concentration, as previously reported [18], or that the differences in half-life between BUN and Alb together with wide variations in pre- and postpartal sampling periods (approximately 2 months) might have affected the relationship with reproductive performance in our region.

In conclusion, the retrospective surveillance findings suggest that dietary remedies such as protein and energy supplementation based on MPT results may be an alternative strategy for reducing excessive usage of hormones for treatment of reproductive failures in the cattle breeding industry. Further field evaluations/experiments are warranted to evaluate the relationship between the metabolism of protein and/or energy and reproductive performance in breeding cattle within our region.