Exposure to chronic light-dark phase shifts during the prepartum nonlactating period attenuates circadian rhythms, decreases blood glucose, and increases milk yield in the subsequent lactation.

Maintaining metabolic balance is a key factor in the health of dairy cattle during the transition from pregnancy to lactation. Little is known regarding the role of the circadian timing system in the regulation of physiological changes during the transition period. We hypothesized that disruption of the cow's circadian timing system by exposure to chronic light-dark phase shifts during the prepartum period would negatively affect the regulation of homeostasis and cause metabolic disturbances, leading to reduced milk production in the subsequent lactation. The objective was to determine the effect of exposure to chronic light-dark phase shift during the last 5 wk prepartum of the nonlactating dry period on core body temperature, melatonin, blood glucose, ß-hydroxybutyric acid (BHB) and nonesterified fatty acid (NEFA) concentrations, and milk production. Multiparous cows were moved to tiestalls at 5 wk before expected calving and assigned to control (CTR; n = 16) or phase-shifted (PS; n = 16) treatments. Control cows were exposed to 16 h of light and 8 h of dark. Phase-shifted cows were exposed to the same photoperiod; however, the light-dark cycle was shifted 6 h every 3 d until parturition. Resting behavior and feed intake were recorded daily. Core body temperature was recorded vaginally for 48 h at 23 and 9 d before expected calving using calibrated data loggers. Blood concentrations of melatonin, glucose, BHB, and NEFA were measured during the pre- and postpartum periods. Milk yield and composition were measured through 60 DIM. Treatment did not affect feed intake or body condition. Cosine fit analysis of 24-h core body temperature and circulating melatonin indicated attenuation of circadian rhythms in the PS treatment compared with the CTR treatment. Phase-shifted cows had lower rest consolidation, as indicated by more total resting time, but shorter resting period durations. Phase-shifted cows had lower blood glucose concentration compared with CTR cows (4 mg/mL decrease), but BHB and NEFA concentrations were similar between PS and CTR cows. Milk yield and milk fat yield were greater in PS compared with CTR cows (2.8 kg/d increase). Thus, exposure to chronic light-dark phase shifts during the prepartum period attenuated circadian rhythms of core body temperature, melatonin, and rest-activity behavior and was associated with increased milk fat and milk yield in the postpartum period despite decreased blood glucose pre- and postpartum. Therefore, less variation in central circadian rhythms may create a more constant milieu that supports the onset of lactogenesis.

Characterization of the analytic performance of an electrochemical point-of-care meter for measuring ß-hydroxybutyrate concentration in blood and plasma from periparturient dairy cattle.

BACKGROUND: The Precision Xtra electrochemical meter is widely used to measure blood ß-hydroxybutyrate concentration (BHBb) in dairy cattle. The meter uses an algorithm optimized for human blood that assumes the HCT in cattle is the same as in people, and that intra-erythrocyte ß-hydroxybutyrate (BHBe) and plasma ß-hydroxybutyrate (BHBp) concentration are equivalent. OBJECTIVES: The first objective was to characterize the analytic performance of the meter for measuring BHBb and BHBp in dairy cattle. The second objective was to characterize the influence of HCT and sample temperature on BHBp concentration measured by the meter. METHODS: Blood and plasma samples were obtained from 106 periparturient Holstein cattle and 15 lactating Holstein cows with experimentally induced electrolyte and acid-base imbalances. Meter performance was evaluated using Deming regression and Bland-Altman plots. Multivariable linear regression was used to determine the effect of HCT and sample temperature on BHBb and BHBp concentration, respectively. RESULTS: The meter was linear up to BHB = 3.0 mmol/L as measured by the reference method, equivalent to meter values for BHBb > 4.5 mmol/L and BHBp > 5.2 mmol/L. An increase in HCT resulted in higher BHBb concentration. This result was partially explained by BHBe being much lower than BHBp. Changes in sample temperature caused a linear change in measured BHBp whenever BHBp > 3.0 mmol/L. CONCLUSIONS: Meter accuracy was markedly dependent on the BHBe-to-BHBp ratio and consequently the HCT. Therefore, the algorithm used by the meter should be revised when applied to bovine blood for improved accuracy.

Clinical utility of calf front hoof circumference and maternal intrapelvic area in predicting dystocia in 103 late gestation Holstein-Friesian heifers and cows.

The objective of this study was to determine the clinical utility of measuring calf front hoof circumference, maternal intrapelvic area, and selected morphometric values in predicting dystocia in dairy cattle. An observational study using a convenience sample of 103 late-gestation Holstein-Friesian heifers and cows was performed. Intrapelvic height and width of the dam were measured using a pelvimeter, and the intrapelvic area was calculated. Calf front hoof circumference and birth weight were also measured. Data were analyzed using Spearman's correlation coefficient (rs), Mann-Whitney U test, and binary or ordered logistic regression; P < 0.05 was significant. The calving difficulty score (1-5) was greater in heifers (median, 3.0) than in cows (median, 1.0). Median intrapelvic area immediately before parturition was smaller in heifers (268 cm(2)) than in cows (332 cm(2)), whereas front hoof circumference and birth weight of the calf were similar in both groups. The calving difficulty score was positively associated with calf birth weight in heifers (rs = 0.39) and cows (rs = 0.24). Binary logistic regression using both dam and calf data indicated that the ratio of front hoof circumference of the calf to the maternal intrapelvic area provided the best predictor of dystocia (calving difficulty score = 4 or 5), with sensitivity = 0.50 and specificity = 0.93 at the optimal cutpoint for the ratio (>0.068 cm/cm(2)). Determining the ratio of calf front hoof circumference to maternal intrapelvic area has clinical utility in predicting the calving difficulty score in Holstein-Friesian cattle.

Dexamethasone-induced exophthalmos in a group of Holstein calves.

Six Holstein calves were evaluated for progressive exophthalmos. The affected calves were receiving daily injections of dexamethasone (30 microg/kg subcutaneously twice daily) as part of a metabolic study. The control calves did not exhibit exophthalmos. The ocular examinations were normal except for the exophthalmos, which ranged from mild to marked in severity. Upon postmortem examination, marked deposition of retrobulbar adipose tissue was noted in the affected calves. Dexamethasone administration appears to increase deposition of retrobulbar adipose tissue resulting in a progressive exophthalmos.