Recently, a question from a Canadian veterinarian caught my attention because it is never heard in the U.S.: "What is the most economical level of understocking for a dairy herd?" Obviously I thought it had to be a typo – no one ever considers understocking their pens! However, the Canadian system of managing their milk supply does make it economical to understock, unlike the U.S. situation.
As I wrote out my reply, it occurred to me that there may well be nutritious "food for thought" for dairy producers despite the prevailing paradigm of overcrowding. Although overcrowding may improve the economic returns on facility investments, it also constrains the cow's ability to practice natural behaviors. What does the cow tell us when she finds herself in understocked pens? What are the economics of under-crowding?
There's not much information I'm aware of that demonstrates a positive performance response to less than 100% stocking rate of stalls or bunk space. Certainly, there are insufficient production data from well-controlled studies to apply sound economics. Of course the economic benefits of understocked transition pens are well established (where the target is more like 80% rather than 100% stocking density), but what about under-crowded production pens?
Studies to date show that providing more than the usually recommended 24 inches of bunk space per cow (such as 30 in./cow), or greater than one headlock per cow, improves the percentage of cows that can eat within the first hour after feed delivery, although dry matter intake was not measured in these studies. British Columbia researchers compared 20 versus 40 inches of bunk space per cow and observed 57% fewer aggressive interactions while eating and 24% greater feeding activity after fresh feed delivery, especially for the subordinate cows (no surprise). One could infer a health or productive benefit from this enhanced feeding activity, especially for more subordinate cows in a pen, but quantifying an economic impact isn't possible with the limited information published to-date.
Does under- or over-crowding the feed bunk influence the cow's decision-making process? A recent study found that about 40% of subordinate cows preferred to eat a less palatable grain mix alone rather than compete with a dominant cow for more palatable grain – even with 30 in./cow of bunk space available. I would also throw in my personal observation from when I was in Nebraska that several dairy farms with 6-row barns experienced better milk production when they managed their pens at about 85% stocking density (stall basis) which essentially corrected for the overcrowding at the bunk inherent in a 6-row design. Most studies I have read found little to no effect on lying time when stalls are understocked (i.e. less than 1 cow per stall). It is still an open question what happens when both the stalls and bunk space are understocked.
Finally, there is a European study that examined the milk production variance among 47 dairy herds and they found that stocking density of stalls alone explained about 1/3 of the variation in milk yield among farms. Between 80 and 120% stall stocking density the herds averaged about 64 pounds/cow/day. At 80% or below, herds averaged about 68 pounds/cow. There was considerable variation among farms, so I would take this milk data with a grain of salt, but it does indicate the potential for a positive milk response with under-crowded cows.
So, does understocking make sense? Economically, no. But we need to think about what the natural cow responses are telling us when there is super-abundant access to stalls and feed (>1 headlock or 24 inches of bunk space per cow; >1 stall per cow). Our industry may well benefit if we can figure out strategies that allow greater access to resources for the individual cow housed in a group (especially the subordinate cow), but within an economical management system.
Located in northern New York, the William H. Miner Agricultural Research Institute conducts research programs that apply basic science to contemporary problems confronting the dairy industry, with a focus on the crop-animal-environment interface, and cow comfort and behavior.