However, it seems that we have forgotten the lessons learned the last time the industry went through a cycle of overstocking triggered by low milk prices. Perhaps now is a good time for a reminder of the many negatives that are associated with this approach.
Short-term gain can turn into long-term pain
There is a mountain of evidence that demonstrates that negative things happen to cows when we overstock freestall pens. It is also true that most overstocking research is not conducted on the numbers of cows we see on commercial dairy farms, and it is done for a limited duration – leaving some gaps in our knowledge. However, we’ve observed that the negative impacts are rarely immediate – they occur over months to years, rather than days to weeks – and for that reason it is sometimes difficult for producers to associate cause with effect. It is also the reason why there is so little quantitative research on the economic impacts of overstocking.
The industry-wide conversation has shifted since the last cycle of overstocking from “How much overstocking can we get away with from an economic perspective?” to “How much overstocking can we tolerate before we significantly negatively impact cow welfare?” The generally accepted tolerable level of overstocking at 120% is a flawed approach built around the prior economic-focused viewpoint and lack of peer-reviewed science at the time.
We now have studies that have looked at the effects of overstocking. Here’s what they found:
Cows reduce lying time when we overstock. At one-and-a-half cows per stall, cows lose approximately 15% of their lying time compared to one cow per stall. This equates to average lying time being reduced from an optimal 12 hours per day (h/d) to 10.2 h/d – the same type of effect we see moving cows from a soft sand-bedded stall to a hard mat surface.
Reduced lying time is associated with increased lameness. At least three studies have shown that reduced lying time of 0.5 to 2 h/d early in lactation is associated with increased risk for hoof lesions and lameness three to four months later.
- There are negative impacts on feeding behavior and milking performance when cows are prevented from eating at the feedbunk at the same time. Overstocked cows sort feed, so the first cows to access feed consume a different ration than cows that access the feed later, which can impact milk output and components. For every 10% refusal of long forage particles in the ration, milkfat decreases by 0.1% and milk protein decreases by 0.04%.
Limited feed access leads to adaptations in feeding behavior – cows tend to decrease feeding time, increase feeding rate and consume fewer, longer duration, larger meals with associated less rumination activity. Reduced feed access is also associated with less de novo fatty acid production. Under overstocked conditions, subordinate cows show altered feeding behavior and blood metabolites, including elevated non-esterified fatty acids (NEFA) and signs of insulin resistance during the transition period, which are risk factors for fresh cow disease problems.
Since pen layout impacts stall stocking density and bunkspace differently, these negative impacts begin at around 24 to 30 inches of available bunkspace per cow, depending on the size and age of the cow. For example, increasing stocking density from 200 to 300 cows in a 200-stall, head-to-head 2-row pen reduces the bunkspace from 28 inches to 19 inches per cow.
Overstocking decreases reproductive performance. Shrinking breeding cow bunkspace allowance to less than 24 inches of bunkspace per cow reduces the probability of becoming pregnant by 150 days in milk, while increasing cows per stall reduces conception rate with a 0.1 percentage point reduction for every 1% increase in overstocking. So, a shift from one to one-and-a- half cows per stall would reduce the conception rate by 5 percentage points.
Overstocking decreases milk output per cow. In a study of 47 herds fed the same total mixed ration (TMR), increasing stocking density decreased milk output when compared to herds with stocking densities ranging from 0.6 to 2 cows per stall. This and another study suggest a loss of 1.14 - 1.25 pounds of milk per increase of 0.1 cows per stall, equating to a 5.7-to-6.3-pound reduction in milk per cow per day from one to one-and- a-half cows per stall.
- Overstocking is associated with elevated somatic cell counts, most likely due to an increased bedding bacterium. Stall beds are occupied by cows for more time each day, so bedding bacteria incubate for longer following an exponential growth curve, leading to a greater risk for teat-end contamination. It is common to see an increased number of mastitis outbreaks associated with overstocking, particularly in the summer when thermal stresses can compound the issues.
Other areas of the farm are also impacted when overstocking occurs. Such as:
- Decreased water access and alley area, an increase in manure. When the structure of the pen does not change and stocking density is increased from one to one-and-a-half cows per stall in a 200-stall, head-to-head 2-row pen:
o Accessible water trough perimeter is reduced from approximately 3.6 inches per cow to 2.4 inches per cow – less than the target of 3 to 4 inches per cow, making a reduction in water intake likely.
o Alley area is reduced from 60 square feet per cow to 40 square feet per cow. More cows means more manure but less area per cow in the alleys, leading to deeper manure and more leg and udder contamination, negatively impacting udder health.
o Manure increases in the pen. The increased manure has to be dealt with in the manure handling system, with negative impacts on sand reclamation systems, which struggle to handle the increase in organic solids.
More cows per pen leads to longer milking times in conventional parlor herds and increased fetch rates in automated-milking-system herds. This reduces the time available for rest and feeding and adds more labor time to move cows through the parlor faster or fetch the cows to the robotic milker.
Increasing the number of cows in a barn leads to less air volume per cow – a risk factor for pneumonia in the winter and heat stress in the summer. Each cow we add contributes increased heat and moisture output in the building space. Ventilation systems are designed around sufficient quantities of air exchange per cow. A 200-cow, naturally ventilated pen provides 2,526 cubic feet per cow, which is reduced to 1,684 cubic feet per cow when the same pen houses 300 cows. Ventilation systems will begin to fail when we have insufficient air exchange and too many cows in the barn.
- Other facilities on the farm feel the strain. Each of the cows added to the milking pen must occupy a space in the transition cow facility and produce a calf that must be reared in the rearing facility, which likely did not change to accommodate the added throughput of animals in the milking pens.
Producers see increased milk shipped from the farm, which gives them a bigger milk check in the short term. However, the negative impacts of overstocking creep up in the long term. Lameness rates increase because of lost resting time, milk per cow goes down around 6 pounds or more per cow per day, milkfat and protein is reduced, and somatic cell count increases; thus, the value of the milk shipped is reduced.
Poor fresh cow performance manifests in increased turnover and death rates, along with reduced peak yields. Compounded, fertility losses lead to a shortage of replacements. It may take two years to realize the full impact of the decision to overstock, but these are the signs of overstocking we repeatedly see over time.
It will always be the case that elite managers will find ways to compensate for some of these negative effects, but farms should be managed so the average producer can succeed. Overstocking has a significant negative impact on dairy cow welfare and on our social license to continue to milk cows. No one has to drink milk from cows anymore, and if we continue to do things that are clearly harmful to our cows, like overstocking, why would the general public continue to have faith in the industry’s ability to do the right thing?
Think about the long-term consequences of overstocking when you are tempted to add a few more cows to the pen to reap short-term rewards – they are real and significant for the individual cow and can impact the industry as a whole.
Photo by Mike Dixon.
References omitted but are available upon request. Click here to email an editor.
Nigel B. Cook
- University of Wisconsin – Madison
- School of Veterinary Medicine