The predictable pattern of increased environmental mastitis during late summer and fall could be taking a near-invisible toll on your bottom line. You can plainly see direct costs of clinical mastitis in early-lactation cows: discarded milk, costly treatment and, frequently, reduced milk production for the rest of their lactations. What you may not see is the indirect cost stemming from reduced reproductive performance.

Mastitis has long been suspected of adversely affecting reproduction. Past research has shown that cows with clinical mastitis caused by E. coli don't have normal reproductive cycles. One study suggested the immune response triggered by an E. coli infection could alter hormone and ovary functions. Other work has shown that any clinical mastitis, not just cases due to E. coli, will cause cows to take longer to conceive. Researchers still need to determine how and why this occurs.

While studies done under experimental conditions have convincingly shown that mastitis hurts reproduction, little field work has been done on how clinical mastitis impacts reproduction in working dairy herds. However, a short report in a recent issue of Veterinary Record does shed light on this subject. The report describes a study done by practising British veterinarians who looked at how mastitis was affecting reproduction in some of their clients' herds.

The vets collected information from detailed records of seven dairy herds in their practice. The producers had been tracking clinical mastitis cases as well as routine reproductive information such as calvings, heats and breedings. They routinely recorded cow identities, lactation numbers, dates of mastitis cases and any treatments given.

For the study, the vets matched a clinically infected cow with an unaffected herdmate having the same lactation number and similar calving date. They compared the reproductive performance of these pairs-a commonly used, powerful study technique.

The vets then put pairs of cows into four groups according to when the mastitis case had occurred relative to artificial insemination (AI):

• up to three weeks before breeding;
  
• three to six weeks before breeding;
  
• up to three weeks after breeding;
  
• six weeks after breeding.

Cows contracting mastitis within the three-week period before insemination were only half as likely to conceive compared with the uninfected cows they were paired with. Reproductive performance was the same for cows with and without mastitis in the other three groups.

Both sets of cows-with and without mastitis-were bred about the same time after calving, 81 and 83 days for infected and non-infected cows respectively. This suggests both groups of cows were fertile and likely had the same opportunity to become pregnant had mastitis not occurred.

How mastitis caused this effect is unknown, but this research suggests the negative effect was likely exerted some time during the 21-day breeding cycle just prior to AI. The mastitis case may have reduced conception by affecting the developing oocyte (egg), follicular growth or ovulation-inducing hormones. The British vets described this specific effect of mastitis as "the window of opportunity" to interrupt reproduction.

As environmental mastitis becomes more prevalent in a herd, the probability increases that more early-lactation cases will occur. Herds in Ontario's Sentinel project a few years ago recorded the greatest number of cases in the first 21 days after calving, before breeding starts. However, a significant number of cases were recorded until 70 days after calving. Then, as lactation progressed, the numbers became much more sporadic. With increased environmental mastitis in the summer, the occurrence of such cases in some cows close to breeding is likely.

Most mastitis cases in the British study were treated. That may have reduced the duration of cases and damage to the cows' udders, but treatments close to breedings did not prevent the adverse impact of mastitis on conception. Mastitis prevention would be the only way to improve reproductive performance.

Ontario herds commonly complain about poor reproductive performance, particularly in summer. Many herds frequently adopt solutions such as intensive timed breeding programs without ever identifying the underlying problem and resolving it. For some, perhaps, solving a mastitis problem should come first.

If you have good records of breedings and mastitis cases, you can find out whether mastitis is hurting your herd's reproductive performance. Your vet or dairy herd improvement representative can help you set up a standard system for recording mastitis cases and treatments. Then you can consult your vet to analyse the relationship between mastitis-and other metabolic conditions-and reproductive performance.

Once you understand underlying conditions that cause poor reproductive performance, you can take the specific steps to correct them. On some farms, preventing mastitis during the breeding period may be the best way to improve conception.

Early-lactation cows more prone to udder infections.

Ontario's average bulk milk somatic cell count (SCC) now hovers around 250,000 cells per millilitre annually. Each year it ranges from monthly lows of 200,000 in winter to highs around 300,000 in late summer and fall months. Over the last 10 years, this pattern has become quite predictable.

Every summer and fall, many Ontario herds experience months with more mastitis cases and high SCCs. Bacteria that thrive in the cow's environment-Strep uberis, Strep dysgalactiae, E. coli and Klebsiella-cause many of these seasonal problems.

The timing often coincides with hot, humid weather that favours multiplication and survival of these bacteria in the cow's environment. When environmental bacteria numbers are high, there is a far greater chance they will enter a teat end from manure in bedding or alleys and infect a quarter. An udder infected with environmental bacteria is more likely to progress into a clinical mastitis case with visibly abnormal milk and even a sick cow.

Typically, environmental mastitis outbreaks cause clinical mastitis in early-lactation cows. These animals have a reduced ability to fight off new udder infections. This suggests the cows' resistance must be low and bacterial numbers must be high to cause a seasonal problem.

Reference:

Reduced conception rates associated bovine mastitis during a "window of opportunity". Veterinary Record (2007) 161, 61-62. Perrin I., R.W. Bostelmann and I.M. Sheldon. July 14, 2007.

This article first appeared in the Ruminations column of The Milk Producer Magazine, September, 2007.

November 2007