A recent review of the “Calving Management Manual,” a publication produced by veterinary researchers at Colorado State University, reminded me of the popular practice that dairy producers sometimes borrow from cow-calf producers. It involves altering the time of day when cows are fed in order to shift the calving window to the daytime. In the manual, the authors reviewed three studies that compared early feeding to late feeding in beef herds. When cows were fed after 5 p.m., approximately 80 percent of calves were born during daylight hours.

However, when dairy producers have tried to apply this practice, their success has been limited. For one thing, the feeding schedule of dairy cows differs from beef cows. Dairy cows are fed so that feed is available ad-libitum, whereas beef cows are usually limit-fed.

What can we learn?

I analyzed a year of records from two client dairy herds in Colorado that record their calving times to see what we could learn. This resulted in some interesting observations. In both herds, calvings that took place during the middle part of the winter were evenly distributed around the clock. But, in mid-summer, a dramatic shift took place: Twice as many calves were born between sunrise and 2 p.m.  And, very few were born between midnight and dawn.

The change was not due to labor issues, as both herds have a herdsman on duty all night. No increase in dead calves was found in the first two hours after dawn to indicate failure of staff to intervene. And both herds have very low and consistent dead-on-arrival rates (less than 5 percent in cows and less than 10 percent in heifers).

The difference between winter and summer calving times was greatest among heifers and cows carrying twins. And, when I summarized the gestational age at calving, the same pattern appeared. In my opinion, this indicates that heifers and cows carrying twins are calving pre-maturely — and in a narrower time window — because low blood sugar levels induce the hormone cascade that starts labor.

What the research tells us

Let’s return to the beef research for a moment. The time of the cows’ last feeding seems to predict calving time. In one study, the animals were fed once a day between 5 p.m. and 10 p.m., which would put their blood sugar at a low point starting eight hours after feeding throughout all of the daylight hours until the next feeding that evening. That, in turn, might explain why 85 percent of these animals calved during the day.

Now, let’s shift it back to dairy. In mid-summer, animals often eat the most when it cools down between 5 p.m. and 7 p.m. Add in the fact that intakes for close-up cows are often marginal and we start to see a feeding pattern — brought on by heat stress — that explains the shift in calving times.  

Since the shift in calving times was more prevalent in heifers, it also may indicate that heifers were more extreme in their feeding behavior in these herds that summer. Given that information, it would be interesting to see if a similar change would occur — a clustering of calving times — in timid heifers when, for example, the close-up pen is overcrowded but with neutral climate conditions. If the same results were to occur, the change in calving times could be used to help indicate a management problem or animal stress — in this case, overcrowding of the close-up pen.

Take-home message

I don’t think we want to recommend any practice which limits feed intake in dairy cows — no matter how convenient daylight calving might be. But, perhaps what we can do is start to use the time-of-calving as a tool to gain insight into how we might better optimize pre-partum intakes, or to indicate a feeding problem which otherwise might go unnoticed.

Marquerita B. Cattell is a consulting veterinarian in Windsor, Colo.