Émile Bouchard

FACTORS AFFECTING MILK UREA NITROGEN AND PROTEIN CONCENTRATIONS IN QUEBEC DAIRY COWS

Milk urea nitrogen (MUN), together with percentage milk protein (PROT), are increasingly used as indicators of the protein-energy balance and for monitoring nutrition and diagnosing feeding disorders. The goal of this study was to analyze the effects of parity, milk yield, days in lactation, somatic-cell count, and herd and feeding factors on MUN and PROT. In 10 dairy herds, one milk sample was taken from each of 418 cows, within +/- 2 days of the routine milk-test visit. We used a four-step multiple linear-regression model with backward elimination, including interactions between herd and the different factors. For both dependent variables, there were significant interactions with herd. Herd-specific models were markedly different; however, the daily amount of protein concentrates fed remained (and had a positive coefficient) in seven of 10 herd models for MUN. This factor is easy to record under field conditions and has to be considered in the evaluation of the ration by means of MUN and PROT. Overfeeding of rumen soluble protein can be easily diagnosed and corrected using MUN analyses. The relationships between MUN and PROT in respect of the factors parity, daily milk yield, and days postpartum also vary considerably among herds.

Effect of nonclinical Staphylococcus aureus or coagulase-negative staphylococci intramammary infection during the first month of lactation on somatic cell count and milk yield in heifers

Coagulase-negative staphylococci (CNS) are the most prevalent cause of intramammary infections in heifers around calving, but Staphylococcus aureus should not be ignored because it is also prevalent, contagious, and more likely to persist into lactation. The objective of this study was to determine the effect of a subclinical infection caused by S. aureus or CNS diagnosed during the first month of lactation in heifers on SCC, milk production, and culling risk during the entire first lactation. Data were obtained from a cohort of 50 farms following a mastitis monitoring and control program and subscribing to the animal health record system (DS@HR) through the ambulatory clinic of the Faculté de médecine vétérinaire of the Université de Montréal (St-Hyacinthe, Québec, Canada). This program included routinely collecting a composite milk sample at each farm visit from all recently freshened heifers. A total of 2,273 Holstein heifers were examined. Among the 1,691 heifers meeting the full selection criteria, 90 (5%) were diagnosed with S. aureus, 168 (10%) were diagnosed with CNS, and 153 (9%) were negative (no pathogen isolated). Test-day natural logarithm somatic cell count (lnSCC) was modeled in a repeated measures linear regression model with herd as random effect. The model-adjusted mean lnSCC in S. aureus and CNS groups were significantly higher than in the culture-negative group from 40 to 300 d in milk. At the test-day level, lnSCC in S. aureus and CNS groups were on average 1.2 and 0.6 higher, respectively, than the culture-negative group. A similar model for milk yield showed that mean milk yield was not statistically different between culture groups from 40 to 300 d in milk. The presence of a S. aureus or CNS intramammary infections in the first month of lactation in heifers correlates with future increased SCC over the entire first lactation.

Evaluation of the California Mastitis Test as a precalving treatment selection tool for Holstein heifers

The objective of this study was to evaluate the California Mastitis Test (CMT) and a portable electrical conductivity meter for diagnosing precalving intramammary infection (IMI) in Holstein heifers. A total of 428 dairy heifers from 23 dairy herds were enrolled between 6 and 12 days before the expected calving date from June 2002 to June 2003. Mammary secretions were tested by both diagnostic methods and by bacterial culture for evidence of IMI. California Mastitis Test was considered negative if the score was negative, trace or 1 and was considered positive otherwise. Two cut-off points were evaluated for milk electrical conductivity (>5 and >6.5 mS/cm). From this study, an overall proportion of 69% of heifers had precalving IMI and the overall heifer prevalence of major pathogen IMI was 16.8%. At the quarter level, sensitivity and specificity of CMT (68.9% and 68.4%, respectively) and milk conductivity >5 mS/cm (41.0% and 65.2%, respectively) or >6.5 mS/cm (25.2% and 83.3%, respectively) to identify all IMI were low. However, the heifer level sensitivity and specificity of CMT for major pathogens were 91.0% (81.5-96.6) and 27.5% (22.8-32.6), respectively. Using a cut-off point of 5 mS/cm, the heifer level sensitivity and specificity for major pathogens was 68.7% (56.2-79.4) and 44.1% (38.7-49.6), respectively. A conductivity cut-off value of 6.5 mS/cm decreased the sensitivity and increased the specificity to 53.7% (41.1-66.0) and 59.5% (54.0-64.8), respectively. California Mastitis Test and milk electrical conductivity are not good predictors of major pathogen IMI in heifers during the last 2 weeks before calving. However, the negative predictive values at quarter or heifer level were high and the heifer false negative rate was 6-14% using CMT or conductivity, respectively. Therefore, these measures could be useful for screening out heifers or quarters that are unlikely to have a major pathogen IMI.

Comparison of results for commercially available microbiological media plates with results for standard bacteriologic testing of bovine milk

OBJECTIVE To compare results for 3 commercially available microbiological media plates with those for standard bacteriologic testing of bovine milk. SAMPLE Milk samples from postpartum cows and cows with a high somatic cell count (SCC) or clinical mastitis (CM). PROCEDURES Sample-ready Staphylococcus culture medium (SRSC) plates were used to detect Staphylococcus aureus in milk samples obtained from postpartum cows and cows with a high SCC or CM. Rapid coliform count (RCC) plates were used to detect coliforms in milk samples obtained from cows with CM. Aerobic count (AC) plates were used to detect streptococci in CM samples. Fresh mastitic milk samples were frozen and then thawed to evaluate the effects of freezing for the SRSC and RCC plates. The effects of dilution (1:10) of samples were determined. Agreement of results between the commercially available plates and standard bacteriologic testing was evaluated. RESULTS The ability of SRSC plates to detect S aureus in milk samples was highest with diluted samples from postpartum cows and cows with a high SCC or CM. Sensitivity of the RCC plate for detection of coliforms was highest with diluted mastitic milk samples. The AC plates had a poor positive predictive value for detection of streptococci in mastitic milk samples. Freezing increased S aureus detection. CONCLUSIONS AND CLINICAL RELEVANCE Overall, the SRSC and RCC plates were accurate, were easy to use, and yielded results comparable to those of standard bacteriologic testing for the detection of S aureus and coliforms in bovine milk.

Contextual herd factors associated with cow culling risk in Québec dairy herds: A multilevel analysis

Several health disorders, such as milk fever, displaced abomasum, and mastitis, as well as impaired reproductive performance, are known risk factors for the removal of affected cows from a dairy herd. While cow-level risk factors are well documented in the literature, herd-level associations have been less frequently investigated. The objective of this study was to investigate the effect of cow- and herd-level determinants on variations in culling risk in Québec dairy herds: whether herd influences a cows culling risk. For this, we assessed the influence of herd membership on cow culling risk according to displaced abomasum, milk fever, and retained placenta. A retrospective longitudinal study was conducted on data from dairy herds in the Province of Québec, Canada, by extracting health information events from the dairy herd health management software used by most Québec dairy producers and their veterinarians. Data were extracted for all lactations starting between January 1st and December 31st, 2010. Using multilevel logistic regression, we analysed a total of 10,529 cows from 201 herds that met the inclusion criteria. Milk fever and displaced abomasum were demonstrated to increase the cow culling risk. A minor general herd effect was found for the culling risk (i.e. an intra-class correlation of 1.0% and median odds ratio [MOR] of 1.20). The proportion of first lactation cows was responsible for this significant, but weak herd effect on individual cow culling risk, after taking into account the cow-level factors. On the other hand, the herds average milk production was a protective factor. The planning and management of forthcoming replacement animals has to be taken into consideration when assessing cow culling risks and herd culling rates.

Marginal structural Cox model to estimate the causal effect of clinical mastitis on Québec dairy cow culling risk

Health disorders, such as milk fever, displaced abomasum, or retained placenta, as well as poor reproductive performance, are known risk factors for culling in dairy cows. Clinical mastitis (CM) is one of the most influential culling risk factors. However the culling decision could be based either on the disease status or on the current milk yield, milk production being a significant confounder when modelling dairy cow culling risk. But milk yield (and somatic cell count) are time-varying confounders, which are also affected by prior CM and therefore lie on the causal pathway between the exposure of interest, CM, and the outcome, culling. Including these time-varying confounders could result in biased estimates. A marginal structural model (MSM) is a statistical technique allowing estimation of the causal effect of a time-varying exposure in the presence of time-varying covariates without conditioning on these covariates. The objective of this paper is to estimate the causal effect on culling of CM occurring between calving and 120 days in milk, using MSM to control for such time-varying confounders affected by previous exposure. A retrospective longitudinal study was conducted on data from dairy herds in the Province of Québec, Canada, by extracting health information events from the dairy herd health management software used by most Québec dairy producers and their veterinarians. The data were extracted for all lactations starting between January 1st and December 31st, 2010. A total of 3952 heifers and 8724 cows from 261 herds met the inclusion criteria and were used in the analysis. The estimated CM causal hazard ratios were 1.96 [1.57-2.44] and 1.47 [1.28-1.69] for heifers and cows, respectively, and as long as causal assumptions hold. Our findings confirm that CM was a risk factor for culling, but with a reduced effect compared to previous studies, which did not properly control for the presence of time-dependent confounders such as milk yield and somatic cell count. Cows experienced a lower risk for CM, with milk production having more influence on culling risk in cows than heifers.

Profitability measures as decision-making tools for Québec dairy herds

Abstract: A retrospective cohort study was conducted using production and health records from Québec dairy herds which were integrated with the objectives of calculating different lifetime profitability measures and developing analyses of the impact of selected reproduction and health variables on profitability. The dataset for the study consisted of lifetime records for a total of 13 668 Holstein cows from 113 herds and in cohorts of cows calving for the first time between 2000 and 2009. To calculate profit, all the revenues and costs were accumulated on a lifetime basis, and cumulative lifetime profit and cumulative lifetime profit adjusted for the opportunity cost of postponed replacement were selected for analysis. There were statistically significant effects in both profit measures because of age at first calving, cumulative number of days in milk, and cumulative number of days dry as well as the number of health events accumulated by animal lifetime (clinical mastitis, reproductive problems, and feet and legs problems). The results of lifetime profit measures and management variables with an effect on profit results could help dairy producers in the process of monitoring and making decisions regarding profit goals and herd life of their cows.