Udder health of dairy cows with an extended voluntary waiting period from calving until the first insemination.

Because of economic returns in relation to yearly peak milk, a one year calving interval (CI) is generally advised to dairy farmers as the optimum lactation cycle. The one year CI roughly means a ten month lactation and a two month dry period. As a consequence of this yearly lactation cycle, cows experience multiple transitions every year, including drying off, calving and the start of the next lactation. During these transitions, large changes in both physiology (e.g. calving, onset of lactation) and management (e.g. regrouping, start of milking) are often associated with an increased risk of diseases and disorders. These include clinical mastitis, hypocalcaemia and ketosis, of which about 75% occur within the first month of lactation. One possible solution to reduce the frequency of transitions and associated health disorders is to extend the lactation length and CI.

Extending the voluntary waiting period (VWP) from calving until first insemination is one way of extending the lactation length beyond 305 days, which will result in an extended lactation length and CI. With an extended CI, the risk of diseases per year can be expected to reduce as there will be fewer calving events per year. It has been shown that cows with an extended VWP (150 days) had a lower incidence of metabolic disorders, lower veterinary costs and lower culling rates compared with cows with a short VWP (60 days). Also, by extending the VWP from 40 to 180 days resulted in a greater proportion of cows drying off at lower milk yields, which could reduce udder health problems around calving related to high milk production at dry-off. On the other side of the coin, extending the VWP may also have negative consequences. Cows with an extended CI have more days in late lactation and cows in late lactation have a greater risk for increased somatic cell counts (SCC) related to the decline in milk yield and low tight junction integrity. Also, a lower milk yield could have a lower dilution effect and result in a greater SCC.

The question posed by the authors cited, was what would be the impact of extending the VWP on SCC or incidence of clinical mastitis during the complete lactation, and in the first weeks of the subsequent lactation. The aim of their study was, therefore, to evaluate the effect of three VWP lengths (50, 125, and 200 days) on SCC, SCC elevations and the clinical mastitis incidence during the complete lactation and the first six weeks of the subsequent lactation.

One hundred and fifty four Holstein-Friesian dairy cows were blocked for lactation number, expected milk yield, calving season and breeding value for persistency, and were randomly distributed across three VWP (50, 125, or 200 days, respectively referred to as VWP-50, VWP-125, VWP-200). The cows were monitored from calving until six weeks into the next lactation, or until culling. An elevation of SCC in milk was defined as SCC equal to or more than 200 000 cells per ml after two previous weeks with SCC less than 200 000 cells per ml.

Over the complete lactation, extension of the VWP did not affect SCC elevations and the occurrence of clinical mastitis per lactation or per cow per year. Extension did reduce milk yield in the 9 weeks relative to dry-off, but did not affect SCC, or the occurrence of SCC elevations or mastitis over the same period. There was also no clear effect of VWP length on SCC in the complete lactation, except that multi-lactation cows in VWP-125 had a higher SCC compared with multi-lactation cows in VWP-50. Dry-off antibiotic use per cow per year was lower in VWP-200 compared with VWP-50 for multi-lactation cows. In the first 6 weeks of the next lactation, cows in VWP-200 had a higher SCC compared with cows in VWP-50, with no effect of VWP on the number of elevations of SCC or the occurrence of clinical mastitis. 

In conclusion, the results indicate that extension of the VWP may be used to reduce the frequency of transition periods and the associated use of dry-cow antibiotics with limited impact on udder health.