Evaluation of milk components as diagnostic indicators for rumen indigestion in dairy cows

Stephanie E. Kirchman Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL 32610.

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Pablo J. Pinedo Department of Animal Sciences, College of Agricultural Sciences, Colorado State University, Fort Collins, CO 80523.

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Fiona P. Maunsell Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL 32610.

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Carlos A. Risco Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL 32610.

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G. Arthur Donovan Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL 32610.

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Abstract

OBJECTIVE To identify milk component alterations that might be useful for detecting cows with rumen indigestion.

DESIGN Prospective case-control study.

ANIMALS 23 Holstein cows with rumen indigestion (cases) and 33 healthy cohorts (controls) from 1 herd.

PROCEDURES Cases were defined as cows between 30 and 300 days postpartum with a > 10% decrease in milk yield for 2 consecutive milkings or > 20% decrease in milk yield from the 10-day rolling mean during any milking, abnormally decreased rumen motility, and no other abnormalities. Each case was matched with 2 healthy cows (controls) on the basis of pen, parity, days postpartum, and mean milk yield. Some cows were controls for multiple cases. All cows underwent a physical examination and collection of a rumen fluid sample for pH measurement at study enrollment. Individual-cow milk yield and milk component data were obtained for the 16 milkings before and after study enrollment. Rumen motility and pH and milk components were compared between cases and controls.

RESULTS Rumen motility for cases was decreased from that of controls. Cases had an abrupt increase in milk fat percentage and the milk fat-to-lactose ratio during the 2 milkings immediately before diagnosis of rumen indigestion. Receiver operating characteristic analyses revealed that a 10% increase in the milk fat-to-lactose ratio had the highest combined sensitivity (57%) and specificity (85%) for identifying cows with rumen indigestion.

CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that a positive deviation in the milk fat-to-lactose ratio might be useful for identifying cows with rumen indigestion.

Abstract

OBJECTIVE To identify milk component alterations that might be useful for detecting cows with rumen indigestion.

DESIGN Prospective case-control study.

ANIMALS 23 Holstein cows with rumen indigestion (cases) and 33 healthy cohorts (controls) from 1 herd.

PROCEDURES Cases were defined as cows between 30 and 300 days postpartum with a > 10% decrease in milk yield for 2 consecutive milkings or > 20% decrease in milk yield from the 10-day rolling mean during any milking, abnormally decreased rumen motility, and no other abnormalities. Each case was matched with 2 healthy cows (controls) on the basis of pen, parity, days postpartum, and mean milk yield. Some cows were controls for multiple cases. All cows underwent a physical examination and collection of a rumen fluid sample for pH measurement at study enrollment. Individual-cow milk yield and milk component data were obtained for the 16 milkings before and after study enrollment. Rumen motility and pH and milk components were compared between cases and controls.

RESULTS Rumen motility for cases was decreased from that of controls. Cases had an abrupt increase in milk fat percentage and the milk fat-to-lactose ratio during the 2 milkings immediately before diagnosis of rumen indigestion. Receiver operating characteristic analyses revealed that a 10% increase in the milk fat-to-lactose ratio had the highest combined sensitivity (57%) and specificity (85%) for identifying cows with rumen indigestion.

CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that a positive deviation in the milk fat-to-lactose ratio might be useful for identifying cows with rumen indigestion.

Rumen indigestion and SARA, a related but more severe form of rumen indigestion, are important disease conditions that affect the health and economic performance of dairy herds. Clinical signs associated with rumen indigestion and SARA include a decrease in milk yield and rumen motility and diarrhea.1 In cattle, when the ratio of concentrate to effective fiber in the ration increases, the production of volatile fatty acids can exceed their rate of absorption by the rumen epithelium, causing the rumen pH to decrease. That decrease in rumen pH can cause rumen indigestion and potentially SARA. Subacute ruminal acidosis is defined as a rumen pH < 5.6 for a period ranging from 148 to 243 min/d or a rumen pH < 5.8 for a period ranging from 284 to 475 min/d, whereas rumen indigestion is defined as a disturbance in rumen motility often associated with a decrease in rumen pH that is less drastic than that observed with SARA or some other cause such as the ingestion of mycotoxins in moldy feed.1 Unfortunately, the onset of clinical signs associated with SARA are often delayed until after the period of pathologically low rumen pH, which makes diagnosis and early intervention to control SARA difficult.2,3 In addition to ration composition, rumen disorders can be caused by extended periods between feedings and restricted feeding subsequent to feed delivery errors.4 Rumen disorders can also develop during hot weather, particularly during extended periods of heat stress, because cows tend to avoid eating during the heat of the day and then overeat at night, when the ambient temperature is cooler.4

Monitoring milk fat percentage has been proposed as a diagnostic tool for assessing whether rumen indigestion and SARA are present at the herd level; however, the association between rumen indigestion or SARA and milk fat percentage is inconsistent, and herds with milk fat percentages within reference limits may still have rumen acidosis problems.4,5 Results of a report6 that summarized the findings of 23 studies indicate that many cows with a rumen pH < 5.8 have milk fat percentages that are within reference limits (3.0% to 3.5%), and milk fat percentage is poorly correlated (r = 0.39) with rumen pH. Moreover, the effects of rumen indigestion and SARA on milk production and milk components are usually transient and variable.3,7,8

Currently, the milking parlors on many commercial dairy operations are equipped with systems that can provide real-time data on milk yield and milk components for individual cows. Thus, the purpose of the study reported here was to identify alterations in milk components that could be used to detect cows with spontaneously occurring rumen indigestion. We hypothesized that lactating dairy cows with rumen indigestion or SARA would have detectable changes in milk components prior to the onset of overt clinical signs that would allow those cows to be identified and managed in a way that would improve their health and productivity and mitigate the need for prolonged medical treatment.

Materials and Methods

Animals

The study was conducted during the summer (June 11 to July 19) of 2012 at the University of Florida Dairy Unit. All study procedures were conducted in accordance with the guidelines for animal research and were approved by the University of Florida Institutional Animal Care and Use Committee. Lactating cows between 30 and 300 days postpartum (DIM) were eligible for the study (approx 275 cows). Cows < 30 DIM were excluded from the study because they are at risk for calving-related disorders that could affect rumen function and feed intake, which could confound the outcome measures. All cows were housed in sand-bedded free-stall barns at 90% to 100% capacity and milked twice daily. Cows were fed a total mixed ration that was designed to meet or exceed National Research Council9 nutritional requirements for lactating cows producing 41 kg (90.2 lb) of milk/d. Feed was delivered twice daily at approximately 6 am and 3 pm and was pushed up twice daily between feedings. During the previous year, this herd had a lactational incidence risk of rumen acidosis or rumen indigestion of 49%, with approximately half of the affected cows developing the condition within 30 DIM.

Sample size calculation

Milk fat percentage is the most variable milk component and was used as the basis to determine the necessary sample size for the study. A difference in milk fat percentage from 3.7% (herd mean) to 3.4% with an SD of 0.4% was used for the calculation. Results indicated 24 cows with a rumen disorder would be sufficient to detect measurable changes in milk fat percentage within the defined parameters. Because milk protein and lactose percentages are less variable than milk fat percentage, the calculated sample size was considered sufficient to detect clinically relevant changes in those components as well.

Measurement of milk yield and components

A herd management systema was used to collect milk yield and milk component data for individual cows. The system included the installation of a milk meter and analysis laboratory on each milking unit and was calibrated as described10 on a monthly basis by technicians who were trained by and worked for the manufacturer of the system. For each cow during each milking, the meter measured the amount of milk produced, and the analysis laboratory measured milk components (fat, protein, lactose, and somatic cell count).11 The system maintained 10-day rolling means for milk yield and milk components for each cow in the herd and calculated deviations from those respective means at each milking. Those deviations were reported as positive or negative percentages and were used as criteria to sort specific cows into a pen (ie, health-check pen) for further evaluation or treatment via automated gates as they exited the milking parlor.

Study design

The study was designed as a prospective case-control study. Cases were cows that developed rumen indigestion, which was diagnosed on the basis of standardized definitions prepared by the herd veterinarians and specified in the standard operating procedures for the herd. For an individual cow, rumen indigestion was defined as a > 10% decrease in milk yield for 2 consecutive milkings or a milk yield during any milking (morning and evening milkings were assessed independently) that was decreased by > 20% from the 10-day rolling mean milk yield, a subjective decrease in rumen motility as assessed by rumen auscultation with or without abnormal fecal consistency, and the absence of abnormalities in organ systems other than the digestive tract.

Cows were evaluated on a daily basis immediately after the morning milking by trained herd health technicians. Cows identified with potential rumen indigestion by the herd health technicians were evaluated within 1 hour by a herd veterinarian to confirm the diagnosis. Each case was matched with 2 healthy cows (controls) that were housed in the same pen as the case (ie, had access to the same ration as the case) on the basis of parity, DIM (± 10 days), and mean milk yield (± 4.5 kg [10 lb]). All controls were determined to be healthy on the basis of results of a physical examination at the time of study enrollment, which was performed within 1 to 2 hours after the associated case was identified.

Data collection

At study enrollment, each case and control underwent a physical examination and collection of a rumen fluid sample. Physical examination included assessment of rectal temperature, heart rate, respiratory rate, rumen motility, fecal consistency, urine pH, and presence of urine ketones. Rumen motility was subjectively assessed by auscultation of the rumen via the left paralumbar fossa with a stethoscope, and data recorded included the number of rumen contractions per minute, strength of contractions (weak, moderate, or strong), and presence of gas. Data for the number and strength of contractions were combined to generate a rumen motility index that ranged from 0 (no rumen motility) to 6 (≥ 2 strong rumen contractions/min). Fecal consistency was scored on a 4-point scale, where 0 indicated normal stacked feces with no splash and 3 indicated feces with a consistency of water.

In conjunction with the physical examination, a rumen fluid sample was collected by use of weighted ororuminal probe.b All rumen fluid samples were obtained 1 to 4 hours after feeding. The first 200 mL of rumen fluid retrieved was discarded to minimize saliva contamination, and a minimum of 100 mL was collected for pH measurement and diagnostic purposes. The pH of the fluid was measured immediately after collection with a portable electronic pH meter,c which was calibrated twice each day before testing rumen fluid samples. The measurements obtained by the portable pH meter were validated by use of a previously described12,13 protocol. Briefly, after the second calibration, the pH for each of 3 buffer solutions (pH, 4, 7, and 10) was measured to verify that the meter was reading the correct pH. After the pH of the rumen fluid samples was determined each day, the meter was used to measure the pH of 2 standardized solutions (pH, 5.6 and 5.8) to further validate that it was correctly measuring the pH. For the purpose of this study, a rumen pH < 5.8 was considered indicative of SARA.14 Following completion of the physical examination and collection of the rumen fluid sample, all cases were orally administered magnesium oxide in accordance with the herd's standard operating procedures.

Milk yield and milk component data were collected for each case and its 2 controls for 16 milkings (8 days) prior to and after diagnosis of rumen indigestion (ie, study enrollment).

Statistical analysis

Demographic and physical examination variables at the time of study enrollment (baseline) were compared between cases and controls by use of ANOVA. For each milking, the milk fat-to-protein and milk fat-to-lactose ratios were calculated by dividing the milk fat percentage by the milk protein and milk lactose percentages, respectively. Because the herd management system reported milk yield and milk component data as percentage deviations from the respective 10-day rolling means, that is how those data were analyzed. The interval between the evening and morning milkings was longer than that between the morning and evening milkings; thus, individual-cow milk yield during the morning milking was consistently higher than that during the evening milking. Therefore, morning milking data were analyzed separately from evening milking data. A repeated-measures ANOVA was developed for each dependent variable (milk yield; milk fat, protein, and lactose percentages; and milk fat-to-protein and milk fat-to-lactose ratios). The model included fixed effects for rumen indigestion (yes or no; ie, case status), parity, time, and the interaction between rumen indigestion and time to account for repeated measures within cows. Assessment of a plot of the within-subject covariance indicated that covariance decreased as the interval between 2 times increased; therefore, an autoregressive covariance structure was specified for the repeated-measure models.

For dependent variables that differed significantly between cases and controls, ROC analysis was used to determine the optimal threshold values for diagnostic use. Briefly, the sensitivity was plotted versus the false-positive rate (1–specificity) for various cutoffs of the dependent variable of interest to determine the optimum threshold value for distinguishing between cases and controls (ie, for distinguishing cows with rumen indigestion from those without rumen indigestion).15,16 The ROC analysis provides the area under the curve, which is a global summary statistic of diagnostic accuracy and is equivalent to the probability that a randomly selected individual with a positive status (ie, affected individual) has a greater test value than a randomly selected individual with a negative status (ie, unaffected individual).17 All statistical analyses were performed with statistical analysis software,d and values of P < 0.05 were considered significant.

Results

Cows

Twenty-seven cows with rumen disorders (cases) and 39 healthy cows (controls) were initially enrolled in the study. Multiple cows with rumen indigestion were identified in the same pen on some days, and because of the extensive matching criteria, some healthy cows served as controls for multiple cases. Four cows with rumen disorders were subsequently removed from the study because they did not meet the definition for a case (1 did not have a milk yield decrease, whereas the other 3 cows had a prolonged [≥ 3 days] period of decreasing milk yield prior to diagnosis instead of the prerequisite > 10% decrease in milk yield for 2 consecutive milkings or > 20% decrease in milk yield from the 10-day rolling mean at any milking). Of the 39 control cows, 6 were excluded from the study when their associated case was removed from the study. Thus, 23 cases and 33 controls were included in the analyses.

Comparisons between cases and controls

At study enrollment (baseline), the mean DIM (P = 0.688), milk yield (P = 0.615), and parity (P = 0.671) did not differ significantly between cases and controls (Table 1), which was expected given that the controls were matched with the cases on the basis of each of those variables. All variables associated with rumen motility (rumen contraction number, rumen contraction strength, and rumen motility index) were significantly greater for controls than for cases. The mean rumen pH for controls (6.25) was significantly (P = 0.006) less than the mean rumen pH for cases (6.47). The rumen pH was < 6.0 for 3 cases and 4 controls, but was not < 5.8 for any cow (ie, SARA was not diagnosed for any cow).

Table 1—

Demographic and physical examination variables at the time of study enrollment (baseline) for 23 Holstein cows with rumen indigestion (cases) and 33 healthy cohorts (controls).

VariableCasesControlsP value
DIM204 ± 61211 ± 610.688
Milk yield (kg)*30.4 ± 11.831.2 ± 12.40.615
Parity1.47 ± 11.61 ± 1.20.671
Rumen contraction (No./min)1.74 ± 0.62.06 ± 0.40.036
Rumen contraction strength1 (0–2)2 (1–3)< 0.001
Rumen motility index1 (0–3)6 (4–6)< 0.001
Rumen gas§1 (0–1)0 (0–1)< 0.001
Fecal consistency0 (0–3)0 (0–1)0.005
Rumen pH6.47 ± 0.36.25 ± 0.20.006

Values represent the mean ± SD or median (range). Cases were defined as cows between 30 and 300 DIM with a > 10% decrease in milk yield for 2 consecutive milkings or > 20% decrease in milk yield from the 10-day rolling mean during any milking, abnormally decreased rumen motility, and no other abnormalities. Cows that met the case criteria were evaluated immediately after the morning milking by trained herd health technicians to identify potential cases, which were confirmed by a herd veterinarian within 1 hour. Each case was matched with 2 controls on the basis of pen, parity, DIM (± 10 days), and mean milk production (± 4.5 kg [10 lb]). Controls were determined to be healthy on the basis of results of a physical examination and were enrolled in the study within 2 hours after the associated case was identified. Multiple cases were identified in the same pen on some days, and because of the extensive matching criteria, some healthy cows served as controls for multiple cases.

10-day rolling mean immediately prior to study enrollment.

Rumen contraction strength was subjectively assessed on a 3-point scale where 1 = weak, 2 = moderate, and 3 = strong.

Rumen motility index represented a combination measure for the number and strength of rumen contractions and was assessed on a 7-point scale, where 0 = no rumen motility and 6 = ≥ 2 strong rumen contractions/min.

Rumen gas was assessed as 0 (absent) or 1 (present).

Fecal consistency was assessed on a 4-point scale, where 0 = normal stacked feces with no splash, 1 = loose stacked feces with no or minimal splash, 2 = loose stacked feces with evident splash, and 3 = feces with a consistency of water.

The absolute values of the mean percentage deviations for milk yield (P < 0.001), milk fat percentage (P = 0.001), and the milk fat-to-lactose ratio (P ≤ 0.001) for cases were significantly greater than those for controls during both the evening and morning milkings immediately prior to diagnosis (Table 2). Because deviation in milk yield was used as a diagnostic criterion for the identification of cases, it had the largest magnitude of deviation between cases and controls (Figure 1). For the cases, the mean milk fat percentage during the evening (3.90%) and morning (3.84%) milkings immediately prior to diagnosis was significantly (P = 0.001) increased by 11% from the 10-day rolling mean milk fat percentage (approx 3.5%). Conversely, during univariate analysis, the mean milk lactose percentage for cases did not differ significantly (P = 0.084) from the 10-day rolling mean milk lactose percentage at either the evening or morning milking prior to diagnosis. However, the mean milk fat-to-lactose ratio for cases was increased from the 10-day rolling mean milk fat-to-lactose ratio by 13% (P = 0.001) and 15% (P < 0.001) during the evening and morning milkings, respectively, prior to baseline (Figure 2). The mean milk protein percentage and milk fat-to-protein ratio did not differ significantly between cases and controls at any time before or after diagnosis (data not shown).

Figure 1—
Figure 1—

Mean ± SE milk yield during the 16 milkings before and after rumen indigestion was diagnosed (study enrollment; milking 0) for 23 Holstein cows (cases; dashed line) and 33 healthy cohorts (controls; solid line). Cases were defined as cows between 30 and 300 DIM with a > 10% decrease in milk yield for 2 consecutive milkings or > 20% decrease in milk yield from the 10-day rolling mean during any milking, abnormally decreased rumen motility, and no other abnormalities. Cows that met the case criteria were evaluated immediately after the morning milking by trained herd health technicians to identify potential cases, which were confirmed by a herd veterinarian within 1 hour. Each case was matched with 2 controls on the basis of pen, parity, DIM (± 10 days), and mean milk production (± 4.5 kg [10 lb]). Controls were determined to be healthy on the basis of results of a physical examination and were enrolled in the study within 2 hours after the associated case was identified. Multiple cases were identified in the same pen on some days, and because of the extensive matching criteria, some healthy cows served as controls for multiple cases. Cows were milked twice daily in the morning and evening; however, the interval between the evening and morning milkings was longer than that between the morning and evening milkings. Thus, milk yield during the morning milking was consistently higher than that during the evening milking, which contributed to the cyclic fluctuation evident in the graph. Even-numbered milkings were morning milkings and odd-numbered milkings were evening milkings.

Citation: Journal of the American Veterinary Medical Association 251, 5; 10.2460/javma.251.5.580

Figure 2—
Figure 2—

Mean ± SE percentage deviation in the milk fat-to-lactose ratio during the 8 morning (A) and evening (B) milkings before and after study enrollment (milking 0) for the cows of Figure 1. Percentage deviation was calculated as the percentage change during the milking of interest from the 10-day rolling mean for the milk fat-to-lactose ratio. The mean percentage deviation for the milk fat-to-lactose ratio for cases during the morning (milking 0, 15%) and evening (milking −1, 13%) milkings immediately before study enrollment was significantly (P ≤ 0.001) greater than that for controls. See Figure 1 for remainder of key.

Citation: Journal of the American Veterinary Medical Association 251, 5; 10.2460/javma.251.5.580

Table 2—

Milk yield and milk component data for the evening and morning milkings immediately before study enrollment for the cows of Table 1.

 Evening milkingMorning milking
 CasesControls CasesControls 
VariableMean ± SDMean percentage deviationMean ± SDMean percentage deviationP valueMean ± SDMean percentage deviationMean ± SDMean percentage deviationP value
Milk yield (kg)11.4 ± 3.5−2314.5 ± 3.4−4< 0.00111 ± 3.4−3115.6 ± 3.2−3< 0.001
Fat (%)3.90 ± 0.16113.73 ± 0.1740.0013.84 ± 0.16113.59 ± 0.1810.001
Protein (%)3.16 ± 0.1333.08 ± 0.091> 0.5003.05 ± 0.1123.05 ± 0.081> 0.500
Lactose (%)4.72 ± 0.07−24.80 ± 0.050> 0.1004.68 ± 0.06−34.80 ± 0.060> 0.050
Fat-to-protein ratio1.23 ± 0.1271.21 ± 0.084> 0.0501.26 ± 0.1091.18 ± 0.060> 0.050
Fat-to-lactose ratio0.83 ± 0.05130.78 ± 0.0550.0010.82 ± 0.07150.75 ± 0.051< 0.001

An automated herd management system was used to collect milk yield and milk component data for individual cows, and the percentage deviation represented the percentage change for the given variable during the milking of interest from the 10-day rolling mean for that variable. The evening and morning milkings were approximately 12 hours and < 3 hours, respectively, prior to study enrollment for all cows.

See Table 1 for remainder of key.

For the variables included in the analyses, the milk fat-to-lactose ratio had the highest combined sensitivity and specificity for diagnosing rumen indigestion on ROC analysis: a 10% increase in the milk fat-to-lactose ratio had a diagnostic sensitivity of 57%, specificity of 85%, and positive predictive value of 72% for this study population in which the prevalence of rumen indigestion was 41% (23/56).

Discussion

In the present study, all cows with rumen indigestion (cases) had evidence of rumen motility dysfunction and a decrease in milk yield in the absence of abnormalities in other organ systems. Only a proportion of cows that met the milk yield decrease criterion met our case definition for rumen indigestion, which was expected because a decrease in milk yield is not specific and is associated with many health disorders. The increase in milk fat percentage in conjunction with the decrease in milk yield observed for the cases was likely a reflection of body fat mobilization rather than a concentration effect because a similar increase was not observed for milk protein percentage. In other studies,7,8,18 the milk fat percentage is not altered in cows with SARA. However, milk fat percentage is dependent on ration composition, specifically the presence of polyunsaturated fatty acids, to a greater extent than the low fiber-to-grain ratios that are traditionally associated with SARA.19 The decrease in milk fat percentage (milk fat depression) historically reported for cows with SARA was believed to be caused by a shortage of ruminal acetate because, as rumen pH decreases, volatile fatty acid production shifts from acetate to propionate7,8,18; however, milk fat content is not a function of ruminal acetate.20 Incomplete saturation (biohydrogenation) of fatty acids within the rumen results in an increase in certain trans unsaturated fatty acids and inhibits milk fat synthesis in the mammary gland.20 For milk fat depression to occur, the ration must have a high concentration of polyunsaturated fatty acids as well as a low fiber-to-concentrate ratio.20 Because milk fat depression is not observed in cows with experimentally induced SARA, it has been proposed that the microbial response to ruminal acidosis is slow, and multiple episodes of SARA must occur for ruminal biohydrogenation to become substantial enough to cause milk fat depression.4,21

Milk fat of ruminants is comprised of > 400 fatty acids owing to the vast lipid metabolism in the rumen.19 In dairy cows, mobilization of stored body fat typically accounts for < 10% of the fatty acids that comprise milk fat, but that proportion increases as the magnitude of negative energy balance increases.19 In a study21 in which SARA was induced weekly in a group of Holstein cows during a 6-week period, the magnitude of the correlation between milk fat content and extent of rumen acidosis was weaker than the respective correlations (both positive and negative) between specific fatty acid concentrations and the extent of rumen acidosis. Overall, milk fat content decreased over the 6-week study period, and the milk fat percentage on the second day after induction of SARA was consistently higher than that on the seventh day after induction of SARA, which was attributed to the cows adjusting to the ration.21

Given that SARA has not been associated with an increase in milk fat percentage in other studies, the increase in milk fat percentage in conjunction with a decrease in milk yield observed for the cases of the present study might have been caused by something other than rumen acidosis, such as the presence of mycotoxins or mold in the feed. Concentrates, pasture grasses, and forages, especially improperly ensiled forages such as corn silage or haylage, are potential sources of mycotoxins for dairy cows. Milk production is negatively associated with the consumption of mold-contaminated silage, with decreases in milk yield of > 15% reported.22

The milk lactose percentage for cases was decreased from the 10-day rolling mean milk lactose percentage for those cows; however, that decrease was not significant, nor did it differ significantly from the milk lactose percentage of controls. We believe the decrease in milk lactose percentage was a consequence of a decrease in rumen function and feed intake, which resulted in insufficient absorption of energy substrates by the rumen. Insufficient energy absorption may have limited the availability of blood glucose for lactose synthesis as well as milk secretion. The osmotic pressure of milk is primarily controlled by lactose; thus, a decrease in the milk lactose percentage decreases the amount of water in the milk and overall milk yield.23 Results of a 1931 study24 indicate that withholding food from Holstein cows causes alterations in milk yield and milk components similar to those observed in the present study (ie, a decrease in milk yield and milk lactose percentage and increase in milk fat percentage).

In the present study, results of the ROC analysis indicated that the milk fat-to-lactose ratio was the best milk component measure for diagnosing rumen indigestion. The null standard, or chance diagonal, of the ROC curve is indicative of a test that has no ability to distinguish between individuals with and without a certain condition.15 An ROC curve that lies above the chance diagonal has some diagnostic ability, which becomes more accurate as it gets closer to the upper left hand corner of the graph.15 The ROC curve for the milk fat-to-lactose ratio was closest to the upper left hand corner when the percentage deviation was 10%, which suggested that cutoff may be clinically useful. However, there may be conditions or diseases other than rumen indigestion that cause a similar change in the milk fat-to-lactose ratio of dairy cows, and use of the milk fat-to-lactose ratio as an indicator of disease in dairy cows requires further investigation.

The rumen pH in healthy dairy cows ranges between 6.2 and 7.2,1 and in the present study, we designated a rumen pH of < 5.8 as diagnostic for SARA. On the basis of that cutoff, SARA was not diagnosed in any of the cows evaluated in this study, and only 3 cases and 4 controls had a rumen pH < 6.0. However, evaluation of rumen pH is challenging because it can vary by 0.5 to 1.0 over a 24-hour period25 and is dependent on the duration since feeding, with the nadir in rumen pH occurring 5 to 8 hours after feeding for cows fed a total mixed ration.5,26,27 In this study, rumen fluid samples were obtained 1 to 4 hours after feeding; therefore, we may have missed the rumen pH nadir for most cows. Sixteen of the 23 cases had ≥ 20% deviation in milk yield from the 10-day rolling mean during the evening milking prior to study enrollment, which suggested that the rumen pH nadir occurred the day before rumen indigestion was diagnosed. Also, an ororuminal probe was used to obtain rumen fluid samples. It is possible that the rumen fluid samples were not obtained from the central or caudoventral aspects of the rumen, the ideal location for measurement of rumen pH, because of faulty probe placement.14 Moreover, rumen fluid samples collected by an ororuminal probe are prone to saliva contamination, which makes such samples less reliable for measurement of rumen pH than samples collected by ruminocentesis.5,14 Because of those limitations, a rumen pH ranging from 5.9 to 6.2 has been suggested as a cutoff for the diagnosis of SARA when rumen fluid samples obtained by an ororuminal probe are analyzed.14 Had we defined SARA as a cow with a rumen pH < 6.2, SARA would have been diagnosed in 5 of the 23 cases. Regardless, use of rumen pH to diagnose rumen disorders is tricky and may not be reliable.

Multiple factors can cause rumen disorders in dairy cows. Therefore, the external validity of the results of the present study is dependent on how representative the study population was of potential target populations. Furthermore, the importance of the magnitude of change in the milk fat-to-lactose ratio of affected cows may be dependent on whether SARA is an important contributor to digestive disorders in the target population.

Results of the present study indicated that, compared with healthy cohorts (controls), dairy cows with rumen indigestion (as determined on the basis of a decrease in milk yield and rumen motility; cases) had a significant increase in the milk fat-to-lactose ratio. However, the rumen pH did not differ between cases and controls, which might indicate that we missed the rumen pH nadir in cases. Nevertheless, rumen indigestion could not be attributed to SARA (rumen pH < 5.8) in any of the cases. Cows with a positive deviation in the milk fat-to-lactose ratio need to be evaluated for rumen indigestion to validate the findings of this study.

ABBREVIATIONS

DIM

Days in milk

ROC

Receiver operating characteristic

SARA

Subacute ruminal acidosis

Footnotes

a.

Afimilk Ltd, Kibbutz Afikim, Israel.

b.

Ruminator, Products of Professor Geishauser, Guelph, ON, Canada.

c.

pHTestr 30, Oakton Instruments, Vernon Hills, Ill.

d.

SAS, version 9.4, SAS Institute Inc, Cary, NC.

References

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