An important factor that influences dairy herd productivity is the type and incidence of disease in the herd. The basis of disease control programs includes knowledge of the frequency and biologic effect of disease, and information on the effectiveness of control procedures.
Most studies only report incidence rates of common, easily diagnosed clinical diseases such as mastitis, lameness, milk fever, retained placenta, or displaced abomasum. The frequency of subclinical disease is much more difficult to discern. The cost to obtain subclinical disease information is inflated by the need to use screening tests (eg, culture or somatic cell counts [SCC] for mastitis, fecal culture or ELISA for paratuberculosis) for diagnosis. However, the high cost of subclinical diseases such as mastitis or paratuberculosis may justify the considerable expense of a control program, and veterinarians can play a vital role in designing these programs. There is zero tolerance for some diseases that have serious consequences for public health. The diagnosis of even one case of bovine spongiform encephalopathy, brucellosis, rabies, or tuberculosis in areas thought to be free of those conditions is cause for immediate action.
Increased culling, reduced milk or protein yield, increased adult cow mortality, and reduced reproductive efficiency are all potential results of disease in adult cows. Milk production is often profoundly reduced in cows with clinical disease. The duration of acute clinical disease is often short, but the effects of the disease may persist throughout the entire lactation. Early lactation is the highest risk period for many diseases. Disease during early lactation may reduce peak milk yields and therefore contribute to lower total lactational yields. Through advances in animal husbandry and health management programs, in concert with close involvement of veterinarians, many farms have minimized clinical syndromes associated with infectious and metabolic disease. The trend toward larger units and shrinking profit margins has encouraged a shift toward optimizing herd or group productivity through reduction of subclinical diseases such as ketosis, mastitis, acidosis, and laminitis, which can have a major impact on productivity.
Infectious disease still represents a major source of loss to dairy industries worldwide. In Britain, outbreaks of foot and mouth disease (as well as bovine spongiform encephalopathy) are dramatic examples of the disastrous effects of infectious diseases on productivity. Other serious infectious diseases such as tuberculosis, brucellosis, bluetongue, and vesicular stomatitis continue to affect livestock around the world. In North America, more common infectious diseases that must be actively controlled include the contagious mastitis pathogens Mycoplasma bovis, Staphylococcus aureus, and Streptococcus agalactiae; bovine viral diarrhea; salmonellosis; paratuberculosis; and pneumonia. Excellent control programs have been developed for most of these diseases, but their adoption is quite variable.
The effects of disease on productivity can be direct (such as mastitis causing a profound reduction in milk yield) or indirect (lameness leading to reduced feed intake, thus causing reduced milk yield). Diseases occurring in early lactation can also cause cascading effects that ultimately reduce productivity over the entire lactation. For example, periparturient disorders often are seen as a complex, and cows diagnosed with parturient paresis are at increased risk of retained placenta, complicated ketosis, and mastitis. Cows with dystocia and retained placenta are at increased risk of metritis. Subclinical ketosis leads to increased risk of displaced abomasum and reduced milk production. The best documented direct effect is the effect of mastitis on milk yield. A single case of clinical mastitis can result in a milk yield loss of 300–400 kg/lactation, with variations ranging from negligible to 1,050 kg. Mastitis during early lactation is associated with higher losses (450–550 kg) than cases seen later in lactation.
Losses resulting from subclinical disease are often considerable. The best-described relationship between subclinical disease and productivity is the effect of subclinical mastitis on milk yield. Each 2-fold increase in SCC >50,000 cells/mL caused a loss of 0.4 kg milk/day in primiparous cows and 0.6 kg milk/day in multiparous cows. Total lactational milk yields were estimated to be reduced by 80 kg for primiparous cows and 120 kg for multiparous cows for each 2-fold increase in the geometric mean SCC >50,000 cells/mL. Other subclinical diseases (eg, paratuberculosis) have also been related to reduced productivity.
Diseases that delay or inhibit conception have a negative effect on herd productivity by prolonging the time cows spend in lower-producing stages of lactation, by reducing the number of offspring for replacements or for sale, and by increasing the likelihood the animal will be culled prematurely. Several diseases have been associated with decreased conception rates. The likelihood of conception was reduced by 14%, 15%, and 21% for cows that experienced retained placenta, metritis, or ovarian cysts, respectively. Mastitis, metritis, and ovarian cysts reduced the likelihood of cattle being bred for the first time. Postpartum diseases that prolong negative energy balance in early lactation also have a negative effect on reproductive performance through alterations in hormonal levels.
The effect of disease on longevity has been investigated. A large proportion of cow culling is considered involuntary (driven by disease, injury, or death) rather than for reasons of low production. The premature removal of a cow from the herd reduces lifetime milk yield. Reproductive failure and mastitis are consistently recorded as the top two reasons for culling.