No abstract

The relationship between Dairy Herd Improvement program test-day milk yield and somatic cell counts in milk was analyzed in 34 dairy herds over 3 yr. Estimates of this relationship were more accurate when somatic cell counts were transformed to natural logarithms rather than actual. For 67,707 observations, test-day milk yield decreased with increasing somatic cell count. The decrease of milk yield for second and later lactations, as somatic cell count increased, was greater than for first lactations. When herds were partitioned according to herd milk production (greater than 7700, 6500 to 7700, and less than 6500 kg/yr), decrease of milk yield was linear with increasing somatic cell counts for herds averaging below 7700 kg milk. Regressions were linear, quadratic, and cubic for both parity groups in herds with high production, over 7700 kg/yr. Quarter samples were composited for each cow and cultured. Percentage of infected cows increased as somatic cell counts increased with greater infection rates above 400 X 10(3).

This paper estimates the financial incentive to control paratuberculosis on dairy farms by establishing the level of expenditure that would minimise the total cost of the disease (output losses plus control expenditure). Given the late onset of the clinical signs and the lack of treatments, control was focused on minimising the financial impact of paratuberculosis by adjusting the dairy cow replacement policy. The optimum replacement policies for disease-free herds and infected herds were compared by using dynamic programming. At the standard settings, the disease justified adjusting the culling policy; under constant bioeconomic assumptions, it reduced the expected annuity from milk production under the optimal replacement policy by about 10 per cent (27 pounds sterling per cow annually), a considerably lower figure than for other major endemic diseases that affect dairy cows in the uk. The effect was even less at lower milk prices, suggesting that there is at present little incentive for dairy farmers to put more resources into controlling the disease. However, the incentive could be increased if more information were available about how best to manage the disease under specific farm circumstances. Any effect that paratuberculosis may have on the future demand for milk and hence on milk prices would also be an important consideration.

A study was conducted to determine the persistence of antibiotic residues in milk beyond the recommended withdrawal period. Composite milk samples (n = 122) were collected from 58 lactating dairy cows in the university herd receiving antibiotic treatments for any reason but only when administered as a single drug. Samples were obtained 72 h posttreatment and sampling continued every 24 h until result for antibiotic residue was negative by the Bacillus stearothermophilus disc assay. The antibiotic (n = 7) administered accounted for significant variation in drug persistence; however, route of administration, case number (for cows treated for more than one episode), number of days treated, animal's body weight, lactation number, and daily milk production did not affect drug persistence. Chi-square analysis indicated that 21% of milk samples were positive for residues beyond the recommended withholding period. Milk samples from cows treated with cephapirin and penicillin were the only samples that exceeded recommended withdrawal times. Often doses administered exceeded label directions.

A study was conducted to determine the effectiveness of on-farm tests for detection of antibiotic residues in milk and urine. Composite milk samples (n = 122) were taken from 58 cows receiving antibiotic treatment with a single antibiotic by any route of administration. Samples were obtained 72 h posttreatment, and sampling continued every 24 h until all residue tests showed a negative result for antibiotics. Antibiotic residues were detected by the Bacillus stearothermophilus disc assay, the Delvotest P, and the Penzyme test. Urine of cull cows was collected and tested for residues with the Live Animal Swab Test for antibiotic residues present in slaughter carcasses. No significant difference was found between the Bacillus stearothermobilus disc assay and either on-farm test. Chi-square analysis indicated that with both the Delvotest and the Penzyme test, 17% of samples gave positive results when the disc assay showed negative readings. An inordinately high percentage (69%) of urine samples from cull cows exhibited antibiotic contamination, although all animals had completed the specified withholding periods. Fifteen of these positive results were obtained from primiparous cows that had never been treated with any antibiotic.