Linear body measurements and productivity of subtropical Holstein-Friesian dairy calves.

OBJECTIVE: The study aimed to describe growth parameters and to quantify the association between linear body measurements as predictors of liveweight (LW) of Holstein-Friesian (HF), and HF crossbred dairy calves in Queensland. A secondary objective was to quantify the effect of disease events on LW change. STUDY DESIGN: Longitudinal study. METHODS: Fortnightly LW, hip height (HH), hip width (HW) and heart girth (HG) measurements were recorded from 16 male and 28 female HF calves from birth until weaning. The association between linear body measurement and the effect of a disease event on LW change were explored using linear mixed-effects modelling with random intercepts and random slopes. RESULTS: HG was the best body measure used individually as a predictor of calf LW (R2 = 82%; P < 0.001), while the combined use of HG, HW and HH was the most accurate predictor of calf LW between birth and weaning (R2 = 90%; P < 0.001). HW, average feed intake and total feed intake were significantly affected by disease events (P < 0.05). On average, total average LW loss associated with a single pneumonia event was estimated at 14.6 kg (95% CI = 10.5 to 18.7 kg; P < 0.05). CONCLUSIONS: Calves of this study performed at a level consistent with the previously published reports. Growth performance was significantly compromised by pneumonia. HW was found to be the least predictive individual measure, and the combined use of HH, HW and HG had the most accurate prediction of calf liveweight from birth to weaning.

The effect of a commercial probiotic product on the milk quality of dairy cows.

Probiotics intended to improve plant health and productivity of pastures grazed by dairy cow are becoming commercially available in Australia. Great Land (GL; Terragen Biotech Pty Ltd., Coolum Beach, QLD, Australia) is one such biologic soil conditioner and spray-on probiotic with a label claim of "acting to improve plant health and productivity." The objective of this study was to quantify the effect of GL on the milk quality of cows grazing pasture top-dressed with GL. Lactating dairy cows of mixed age and breed (primarily Holstein-Friesian), in their second lactation or greater, and at least 80 d in milk were enrolled and randomly allocated into 1 of 2 study groups: a treatment cow group (n = 98; cows grazed pasture that was top-dressed with GL according to the product label) and a control cow group (n = 114; cows grazed untreated pasture). As required, both groups were supplemented at the same rate with a mixed ration during the grazing period. Composite milk samples were collected weekly from each cow during the study and analyzed to determine milk components. Milk volumes were recorded at each milking using the herd management software of the study farm. Mean differences in the milk component variables were compared using mixed-effects linear regression models. After controlling for the effect of days in milk, cow lactation, and time since a cow entered the study, the treatment cows produced an average of 1.21 L/cow per day more milk (95% confidence interval: 0.34-2.08 L/cow per day) and more milk protein (0.03 kg/d; 95% confidence interval: 0.01-0.05 kg/d) than the control cows. Pasture cover and pasture consumption did not differ between the GL-treated and the untreated study paddocks grazed by the treatment or control cows. A limited amount of published data have examined the effect of probiotic pasture treatment on the milk quality of dairy cows. This study suggests that application of such products may be beneficial. The mechanisms associated with this type of outcome remain to be investigated.

Respiratory muscle blood flow in the fetal lamb during apnoea and breathing.

We measured blood flow to the respiratory muscles of the fetal lamb using the radioactively-labelled microsphere technique in order to assess whether fetal breathing is an energetically costly activity as has been reported. Diaphragm flow ranged from 6.4-35.2 ml.min-1.100 g-1 during fetal apnoea and rose to 21.1-615 ml.min-1.100 g-1 during fetal breathing (P < 0.02; n = 7). Parasternal muscle flow also increased significantly (P < 0.02) between fetal apnoea and breathing while external and internal intercostal flows did not change. Expressed as a percentage of cardiac output the diaphragm received 0.08-0.28% during apnoea and 0.22-2.2% during fetal breathing. Neither placental blood flow nor fetal O2 consumption increased significantly between fetal apnoea and breathing. We conclude that the levels of perfusion required by the respiratory muscles for breathing in the fetus are inconsistent with fetal breathing costing a large proportion of the fetal O2 budget.

Origin of tonic electrical activity patterns in the diaphragm of the fetal lamb.

The respiratory pattern generator in fetal and postnatal life activates the phrenic nucleus and diaphragm muscle with phasic bursts of activity. In the fetus, diaphragmatic activity is also characterized by tonic activity patterns of unknown origin. We have examined whether such activity is diaphragmatic, or radiated from nearby ribcage muscles, by placing two sets of electrodes side-by-side in the costal portion of the diaphragm in five fetuses. The rationale for this approach is that if tonic activity, radiates to the diaphragm it should be recorded by both sets of electrodes and there should be no delay between the action potentials from each set of electrodes. Of 24 single tonic units identified, 15 were recorded from only one of the two sets of electrodes in the diaphragm. In the 9 tonic units recorded from both sets of electrodes, there was a time delay between the appearance of the action potentials in the two recordings (mean +/- S.E.M. 1.6 +/- 0.2 ms). This is the expected conduction delay along the muscle fibres separating the two electrodes. Since tonic diaphragmatic activity persisted in fetuses with the spinal cord transected rostral or caudal to the phrenic nucleus, we conclude that the spinal cord alone is sufficient to produce the tonic activity recorded from the fetal diaphragm but that the brain may also generate such activity.

Differential recruitment of inspiratory muscles in response to chemical drive.

Changes in the intensity of EMG activity in the costal diaphragm, crural diaphragm and external intercostal muscle during inspiration were assessed in intact, awake lambs following abrupt transitions in the composition of the inspired gases from air to either a hypoxic/hypoxic mixture (10% O2, 90% N2), a hyperoxic/hypercapnic mixture (40% O2, 6% CO2, 54% N2) or a hypoxic/hypercapnic mixture (10% O2, 6% CO2, 84% N2). A regression method was used to compare the dynamic responses of the three muscles over the 10-min period following each transition. The relationship between the dynamic response functions of the costal and crural diaphragm was the same in each of the three experimental conditions, indicating that these separate components of the diaphragm comprise a single functional unit during breathing. The relationship between the dynamic response functions of the external intercostal muscle and the costal diaphragm varied according to the composition of the inspired gas mixture. This result suggests that the central and peripheral chemoreceptors exert differential effects on the activation of the diaphragm and the external intercostal muscles during breathing, consistent with the hypothesis that sensory information from these receptors is processed, at least in part, in parallel pathways which project separately to the phrenic and external intercostal motoneurons.

Indirect relation between rises in oxygen consumption and left ventricular output at birth in lambs.

To examine the relation between increased newborn oxygen requirements and the postnatal rise in cardiac output, we measured left ventricular (LV) output, organ blood flows, and whole-body oxygen consumption using radioactive microspheres in late-gestation sheep fetuses and in the same animals 1 and 4 hours after cesarean section delivery. LV output rose from 264 +/- 23 ml.min-1.kg body wt-1 in fetuses to 444 +/- 33 ml.min-1.kg body wt-1 in lambs at 1 hour after delivery (p less than 0.005) and was unchanged at 4 hours after delivery. This rise in LV output was associated with a more than fourfold increase in the LV flow contribution to tissues situated distal to the ductus arteriosus (fetus, 51 +/- 9 ml.min-1.kg body wt-1; lamb, 226 +/- 22 ml.min-1.kg body wt-1; p less than 0.005), which were mainly perfused by the right ventricle in utero. However, average blood flow to body tissues was similar in fetuses (37 +/- 4 ml.min-1.100 g tissue-1), 1-hour lambs (39 +/- 4 ml.min-1.100 g tissue-1), and 4-hour lambs (40 +/- 5 ml.min-1.100 g tissue-1). Oxygen consumption increased by 58%, from 7.84 +/- 0.43 ml.min-1.kg body wt-1 in fetuses to 12.38 +/- 2.4 ml.min-1.kg body wt-1 in 1-hour lambs (p less than 0.01), and was unchanged in 4-hour lambs.(ABSTRACT TRUNCATED AT 250 WORDS)

Electromyographic changes in the isolated rat diaphragm during the development of fatigue.

To investigate whether the power spectrum of the electromyogram of a fatiguing muscle can be used to infer the degree to which the muscle is fatigued, we recorded isometric tension and two monopolar electromyograms from eight isolated rat diaphragm preparations suspended in an organ bath containing a balanced salt solution. Each preparation was excited with a fixed phrenic nerve impulse pattern made up of a 70-Hz train of impulses of supramaximal voltage delivered for 170 ms with a 500-ms recovery period. Tension fell rapidly over the first 60 s of the fatigue run and more slowly for the remaining 60 s analysed. The duration of extracellular action potentials increased and their amplitude decreased as the tension developed by the diaphragm decreased; conduction velocity along muscle fibres also decreased. The centroid frequency (fcen) of the power spectrum of the first action potential elicited by each train of stimuli decreased rapidly until tension fell to approximately 70% of the initial value; thereafter little change in fcen occurred, although tension continued to fall to 33% of its initial value. Our results demonstrated that under controlled conditions, fcen provided a sensitive index of fatigue in its early stages, but provided no information once fatigue was pronounced.

Breathing at birth and the associated blood gas and pH changes in the lamb.

We examined the relationship between the initiation of breathing at birth and the timing of delivery of the chest in a group of 13 lambs undergoing spontaneous unassisted delivery at term. In 8 of 11 lambs with a diaphragm electromyogram or intrapleural pressure signal the first breath occurred before chest delivery. The first breath was always followed by a period of irregular and often powerful inspiratory efforts. Some of these inspirations were followed by a forceful expiration. A regular respiratory rhythm never developed until the chest had delivered, suggesting that chest expansion is essential for the establishment of rhythm. Although PaO2 increased rapidly after birth, pHa declined and reached its lowest level approximately 20 minutes postnatally suggesting that considerable anaerobic metabolism occurs in the face of adequate arterial oxygenation. The level of respiratory activity in the first 30 min following birth did not appear to be related to arterial PO2, PCO2 or pH. Neither the appearance of the EMG activity of the first breath, nor blood samples taken from 2 lambs simultaneously with the first breath, suggested that the first breath was a gasp initiated by asphyxial blood gases. Our results therefore do not support the current hypothesis that the first breath is a gasp initiated by asphyxia accompanying delivery.

Diaphragm VO2, diaphragm EMG, pressure-time product and calculated ventilation in newborn lambs during hypercapnic hyperpnoea.

We examined the relationship between diaphragmatic EMG (EMGdi) and pressure-time index (Pt'), and diaphragmatic oxygen consumption (VO2di) in 6 newborn lambs quietly breathing air and during 3 levels of hypercapnic hyperpnoea (PaCO2 was equal to 48, 53, or 60 Torr). We also determined minute ventilation (VI), whole body oxygen consumption (VO2), diaphragmatic oxygen consumption (VO2di), and respiratory muscle oxygen consumption (VO2resp). VO2 did not change significantly throughout the experiment. While breathing air VO2di was 0.46 +/- 0.03 ml.min-1, VO2resp was 1.40 +/- 0.22 ml.min-1, and VI was 1.98 +/- 0.11 L.min-1. These values increased significantly during hyperpnoea and reached 3.88 +/- 0.45 ml.min-1, 7.22 +/- 1.12 ml.min-1, and 6.99 +/- 1.03 L.min-1, respectively. EMGdi, and Pt' also increased significantly during hyperpnoea. A strong linear relationship was found between the normalized values of EMGdi and Pt' (r = 0.90), between VO2di and EMGdi (r = 0.88), and between VO2di and Pt' (r = 0.88). The relationship between both EMGdi and VI and PaCO2 was logarithmic. We found that (i) both EMGdi and Pt' correlate well with VO2di, and (ii) VI, VO2di, and EMGdi increased significantly when PaCO2 was greater than 48 Torr.

Blood flow to the respiratory muscles during hypercapnic hyperpnoea in the newborn lamb.

The blood flow to the diaphragm, external and internal intercostal muscles, abdominal oblique muscles, and other rib-cage and abdominal muscles was measured, using radio-labelled microspheres, in 6 newborn lambs quietly breathing in air and during 3 different levels of CO2 induced hypercapnic hyperpnoea (inspired gas containing 4%, 5.5%, or 7% CO2). We also calculated the oxygen uptake of the diaphragm (VO2di). While the lambs were breathing air diaphragmatic blood flow (Qdi, 38.2 +/- 4.0 SEM ml.min-1.100 g-1) was similar to external intercostal muscle blood flow (Qei, 37.1 +/- 8.1 ml.min-1.100 g-1), and both were greater than internal intercostal muscle blood flow (Qii, 24.8 +/- 6.1 ml.min-1.100 g-1; P less than 0.05). During hyperpnoea Qdi, Qei, and Qii were augmented with Qdi equal to 200.1 +/- 12.2 ml.min-1.100 g-1 in 7% CO2 and Qei equal to 88.4 +/- 14.1 ml.min-1.100 g-1 in 7% CO2 (Qdi was greater than Qei, P less than 0.01). Qii was 40.7 +/- 5.6 ml.min-1.100 g-1 in 7% CO2 being less than Qdi (P less than 0.01) and Qei (P less than 0.05). The abdominal oblique muscles also had augmented flow in response to hyperpnoea. The level of hypercapnia that resulted in an augmentation of Qdi (5.5% inspired CO2) was lower than that which augmented Qei and Qii (7% inspired CO2). VO2di was linearly related to Qdi (r = 0.98). Our results suggest that in the newborn lamb the diaphragm is the dominant respiratory muscle in response to hypercapnia.

Origins and regional distribution of blood flow to the respiratory muscles in conscious sheep.

The blood flow to the portions of the diaphragm (sternal, costal, and crural) and to the external and internal intercostal muscles was measured, using radio-labelled microspheres, in 12 young sheep breathing quietly. We calculated total blood flow and oxygen uptake of the diaphragm (VO2di). We also determined the proportion of diaphragmatic and intercostal muscle blood flow that arose from the descending aorta. Diaphragmatic blood flow (Qdi) was 0.62 +/- 0.04% of cardiac output (mean +/- SEM). VO2di was 1.53 +/- 0.15 ml X min-1 X 100 g-1, and was linearly related to Qdi. When standardized for weight Qdi was 28.3 +/- 1.5 ml X min-1 X 100 g-1, significantly greater than both external intercostal muscle blood flow (18.9 +/- 2.6 ml X min-1 X 100 g-1; P less than 0.05), and internal intercostal muscle blood flow (11.4 +/- 1.4 ml X min-1 X 100 g-1; P less than 0.01). Blood flow to the costal portion of the diaphragm was significantly less than blood flow to the crural portions. There was no difference between flow to these portions and to the sternal portion of the diaphragm. The blood supplying the dorsal-most regions of the diaphragm arose predominantly from the descending aorta. The results suggest that the crural portions work at a higher rate than the costal portions under resting conditions.