Clinical mastitis and conception rates*

QM2 (Quality Management • Quality Milk) is the newsletter of Dairy One and Quality Milk Production Services. It appears in Eastern DairyBusiness.

By Julia Hertl, George Cudoc and Gary Bennet, DVM

*Based on “Effects of clinical mastitis caused by gram-positive and gram-negative bacteria and other organisms on the probability of conception in New York State Holstein dairy cows”, by JA Hertl, YT Gröhn, JDG Leach, D Bar, GJ Bennett, RN González, BJ Rauch, FL Welcome, LW Tauer, and YH Schukken; Journal of Dairy Science  93: 1551-1560. USDA (CSREES) Award No. 2005-35204-15714 provided funding.

Fertility plays a key role in successful dairy cow management. Many factors play a role in reproduction problems, including missed estrus and poor estrus expression. To avoid these problems, many farms use ovulation synchronization and planned breeding programs.

Other contributing factors include heat stress, high milk production, and diseases, including clinical mastitis (CM) – the focus of this article. The severity of the CM effect on conception may depend on both the causative agent(s) and when CM occurs, relative to artificial insemination (AI). Causative agents of mastitis can be grouped into Gram-positive vs. Gram-negative classes. Clinical signs, severity and treatment protocols differ for Gram-positive and Gram-negative infections, making this a reasonable distinction.

To address what effects Gram-positive and Gram-negative CM have on probability of conception, we studied data on 55,372 AIs in 23,695 lactations from 14,148 Holsteins in seven New York State herds. Four herds were in central New York; two were in northern New York; and one was in western New York. All lactating cows inseminated at least once between 40 and 90 days in milk were eligible for inclusion in our analysis. Cases of CM were generally detected by milkers or herdspersons. CM signs included a warm, swollen udder, changes in milk consistency, elevated milk electrical conductivity and milk loss. Sick cows were treated based on similar protocols in all seven study farms. Milk samples from udder quarters with CM signs were sent to the Quality Milk Production Services laboratories for microbiological diagnosis.

We developed a model to calculate the probability of conception after AI. We included factors that might affect probability of conception: the cow’s parity, her farm, milk yield, season of AI (winter, spring, summer, fall), insemination attempt (1st, 2nd, 3rd, 4th) of the lactation, CM, and other diseases (milk fever, retained placenta, metritis, ketosis, displaced abomasum).

Our main interest was the effect of CM. Only cases occurring between 42 days before until 42 days after an AI were included in the calculation of the probability of conception. (Outside this range, CM had no effect on probability of conception). We created variables for each CM type (Gram-positive, Gram-negative) in relation to when CM occurred with respect to an AI for these intervals: CM occurring: 36-42 days before an AI; 29-35 days before an AI; 22-28 days before an AI; 15-21 days before an AI; 8-14 days before an AI; 1-7 days before an AI; 0-7 days after an AI; 8-14 days after an AI; 15-21 days after an AI; 22-28 days after an AI; 29-35 days after an AI; and 36-42 days after an AI.

Probability of conception was lower after successive AIs.

(To view the graphs and figure associated with this article, visit For details on how to calculate such probabilities, see “Effects of clinical mastitis caused by gram-positive and gram-negative bacteria and other organisms on the probability of conception in New York State Holstein dairy cows”, by JA Hertl, YT Gröhn, JDG Leach, D Bar, GJ Bennett, RN González, BJ Rauch, FL Welcome, LW Tauer, and YH Schukken; Journal of Dairy Science 93: 1551-1560.)

Gram-positive and Gram-negative CM both lowered the probability of conception. The lowest probability of conception was associated with Gram-negative CM occurring 0-7 d after AI. Probability of conception was lower when CM occurred close to the time of insemination, whether before or after. Gram-negative CM was generally more detrimental than Gram-positive CM, except in the period 29-35 d after AI.

Findings summarized

• 27% of lactations had CM

Escherichia coli most often isolated; Streptococcus spp. and Klebsiella spp. also frequently found

• Of the Gram-classified cases, 47% were Gram-positive and 53% were Gram-negative

• Probability of conception (in all cows) after 1st AI = 0.29; after 2nd AI = 0.26; after 3rd AI = 0.25; after 4th AI = 0.24

• Older cows less likely to conceive

• Summer AIs less successful

• Retained placenta reduced probability of conception

• From 14 days before until 35 days after an AI, CM’s effect varied with both type and timing with respect to AI

• Either type of CM occurring near the time of AI (particularly immediately before and after an AI) significantly reduced probability of conception.

• Gram-negative CM generally more detrimental than Gram-positive CM

Many researchers have found a connection between bacterial infections and poor reproductive performance; our findings appear to corroborate this. The biological mechanisms are complex, and differ between Gram-positive and Gram-negative bacteria.

Our findings can help determine whether it’s worth inseminating a particular cow at a particular time, especially if she has CM (and it’s Gram-negative), or if it’s better to wait until the next cycle, when hopefully the infection will have resolved. Furthermore, by waiting, it may be possible to avoid wasting resources (e.g., semen, technician time, treatment of the CM) on a vain endeavor.

Additional knowledge about CM (its timing with respect to AI, and whether the causative agent is Gram-positive or Gram-negative) seems beneficial in determining why some cows have trouble conceiving. Our findings imply improved mastitis control is associated with improved reproductive performance. This information may also aid management of CM cows before and after AI, thereby helping to improve a dairy’s profitability. Our current research will study management of CM knowledge and profitability in detail.

How to reach us…

Julia Hertl is a research aide for Cornell’s Department of Population Medicine and Diagnostic Sciences. Contact her via e-mail:

George Cudoc, DHI Support, DairyOne, in Ithaca, N.Y. Contact him via e-mail:

Gary Bennett, DVM, is Senior Extension Veterinarian at the Canton Quality Milk lab. Contact him via e-mail:

QMPS is a program within the Animal Health Diagnostic Center, a partnership between the New York State Department of Agriculture and Markets and the College of Veterinary Medicine at Cornell. The QMPS staff of veterinarians, technicians and researchers works with New York dairies to improve milk quality by addressing high somatic cell counts, milking equipment and procedures, and milker training in English and Spanish. QMPS also conducts research and teaching programs.

Reach the four regional QMPS
laboratories at:

Central Lab, Ithaca.

877-MILKLAB (877-645-5522)

Eastern Lab, Cobleskill.


Northern Lab, Canton.


Western Lab, Geneseo.


QMPS website:

Dairy One is an information technology cooperative, providing DHI records services and herd management software to dairies throughout the Northeast and Mid-Atlantic region. A comprehensive laboratory network provides milk quality testing as well as forage, soil, manure and water testing.

Contact Dairy One Cooperative Inc. at 730 Warren Rd., Ithaca, N.Y. 14850. Tel: 800-344-2697. Email: Website: