‘Doc, remind me why I want more old cows’

Mr. S, a great client of ours, said those exact words to me not long ago while one of his cows was dying. She was a dry cow, due to calve soon, and she was down and making the awful groaning and wailing noise cows often make when dying. He said those words because a lot of advisors have been telling him to reduce culling by keeping more older cows. The idea is to reduce herd replacement costs and increase milk production and, in the end, increase profitability. This all makes sense and is probably still a good idea for most dairy producers, but there can be a downside, as Mr. S was seeing that day.
A recent paper in the Journal of Dairy Science, by Lean, et. al., discusses the differences in rates of health disorders in young and old cows. The authors collected information from studies in Australia, Canada and the United States accounting for 28,230 Holstein cows. As one would expect, clinical milk fever was much more common in older cows. The overall rate per lactation was 2.7%, but the odds of clinical milk fever were 3.5, 8.6 and 20.2 times greater for parity 3, 4 and 5 and up versus parity 1, respectively. Over 12% of parity 5 and up cows had clinical milk fever. Most likely this is due to decreased capacity to mobilize calcium from bone with age. This may involve impaired vitamin D metabolism or reduced ability to absorb calcium from the gut as well.
Clinical mastitis also increased more with parity, though the increase was not as dramatic. The overall rate of mastitis per lactation was 17.4%. The odds of clinical mastitis were 1.2, 1.7, 1.9 and 2.5 times greater for parity 2, 3, 4 and 5 and up rather than parity 1, respectively. In our practice, we do not monitor clinical mastitis by lactation, but we do monitor somatic cell count by lactation group. In our dataset, the mean SCC in thousands was 120, 129 and 251 for lactation one, two and three and up, respectively. However, there are a few herds where the SCC did not change by lactation. These herds all have herd average SCCs below 100,000, so most cows in these herds probably never get infected at all. The reason older cows seem to be more susceptible to mastitis is probably just that every infection increases the chance of another, so older cows may not be inherently more susceptible if we can keep them clean.
The odds of a case of lameness by parity compared to lactation one were 1.6, 3.1, 4.3 and 5.6 for parity 2, 3, 4, and 5 and up, respectively. This is probably no surprise to anyone who has worked with dairy cows. However, there is no obvious explanation for this greater risk, except that older cows have more cumulative exposure to concrete and other hard surfaces than younger cows.
The average rate of dystocia was much greater for parity 1 than others, with parity 2, 3, 4 and 5 and up cows having odds of dystocia of 0.55, 0.49, 0.50 and 0.70, compared to lactation one. This is also unsurprising since heifers are not fully mature at first calving. The increase in dystocia for cows in lactation 5 and up may be due to the much higher risk of clinical milk fever, because clinical milk fever can often result in dystocia.
The relative risk factors for metritis by parity showed a similar pattern as for dystocia, with parity 2, 3, 4 and 5 and up having odds of .57, .55, .52 and .59 compared to lactation one. This could be due to the increased risk for dystocia in parity one.
For displaced abomasum, the relative risks were 1.5 ,4.0, 3.7 and 2.9 by parity, so first lactation animals were significantly less likely to get a DA than older cows. It is curious that the highest risk is for parity 3 and not older cows, because hypocalcemia has been postulated to be one of the causes. More likely, the greatest risk factor is excessive fat mobilization before or after calving. The mechanism in those animals may involve inappetence or inflammation resulting from fat mobilization, though it is unclear why older cows would have less fat mobilization.
Clinical ketosis risk was similar, with relative risks of 1.5, 3.9, 3.7 and 4.9 by parity compared to parity 1. The biggest risk for clinical ketosis is likely the presence of another disease.
This paper did not assess risk of poor reproductive performance, but our practice’s data set showed average pregnancy rates of 35%, 31% and 27% for parity 1, 2 and 3 and up. However, there were a number of herds with rates that did not significantly differ by lactation group, so it is possible to achieve similar performance in older cows, and older cows may not be inherently at more risk.
So, if Mr. S has a greater proportion of older cows, he can expect higher milk production, fewer cases of dystocia and metritis but more cases of milk fever and lameness. He may or may not see more cases of mastitis and impaired reproductive performance. The question for all of us is how we manage old cows to not have greater incidence of these conditions. Our clients have shown that this is possible, except maybe for milk fever, but I am betting they will get that figured out as well. For the most part, old cows do not necessarily need to have more problems.
   Bennett is one of four dairy veterinarians at Northern Valley Dairy Production Medicine Center in Plainview, Minnesota. He also consults on dairy farms in other states. He and his wife, Pam, have four children. Jim can be reached at [email protected] with comments or questions.