Stocking Density

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 that 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 us to associate cause with effect. It is also the reason why there is so little quantitative research on the economic impacts of overstocking.

When a pen of freestall-housed cows is overstocked, we decrease the availability of resting spaces, feeding spaces, and access to water. We also reduce the square footage of the pen area per cow and the barn volume per cow – all of these effects have a significant negative impact on the cow.

Let’s consider what happens to cows when we increase stocking density within the commonly observed range from 1 to 1.5 cows per stall.

• In a review of lying time studies, Tucker et al. (2021) found that cow lying times are reduced when we overstock. At 1.5 cows per stall, cows lose ~15% of their lying time compared to 1 cow per stall. This equates to average lying time being reduced from an optimal 12 hours per day to 10.2 hours per day – the same type of effect that we see from moving cows from a soft sand bedded stall to a hard mat surface. Since resting time is so important for the cow, any deliberate reduction in a cow’s ability to rest is significant. And just because we build and provide one stall per cow, it does not mean that all stalls are used identically. With small 46-inch (117 cm) wide stalls, it is common for the rumen and legs of one Holstein cow to overlap adjacent stalls, potentially reducing the use of these stalls. For that reason, stalls should be sized appropriately to the size of the cows occupying them.
• Reduced lying time is associated with increased lameness. Proudfoot et al. (2010), Sepulveda-Varas et al. (2018), and Omentese et al. (2020), showed that reduced lying time of 0.5 to 2 hours per day early in lactation was associated with an increased risk for hoof lesions and lameness 3 to 4 months later. Numerous field experiences and studies have shown that when we make improvements to cow comfort, lying times increase and lameness decreases over months to years (Morabito et al., 2017).
• When all the cows are prevented from eating at the feed bunk at the same time, there are negative impacts on feeding behavior and milking performance. Overstocked cows sort feed (Leonardi and Armentano, 2003), 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, milk fat decreases by 0.1% and milk protein decreases by 0.04% points (Miller-Cushon and DeVries, 2017). 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 (DeVries, 2019). Reduced feed access is also associated with less de-novo fatty acid production (Woolpert et al., 2017). Under such conditions, subordinate cows show altered feeding behavior by not going back to the bunk later once stocking density at the bunk has decreased, and have different blood metabolites, including elevated non-esterified fatty acids (NEFA) and signs of insulin resistance during the transition period (Huzzey et al., 2012), 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 (61 to 76 cm) 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 (71 to 48 cm) per cow.
• Overstocking decreases reproductive performance. Shrinking breeding cow bunkspace allowance reduces the probability of becoming pregnant by 150 DIM, with optimal performance at ~24 inches (~61 cm) of bunkspace per cow (Caraviello et al., 2006). Another study showed that increasing cows per stall reduced conception rate with a 0.1 percentage point reduction for every 1 percent increase in overstocking (Schefers et al., 2010). So a shift from 1 to 1.5 cows per stall would reduce the conception rate by 5% points (e.g. decreasing from 40% to 35%).
• Overstocking decreases milk output per cow. In a study of 47 herds fed the same TMR, increasing stocking density decreased milk output, comparing across herds with stocking densities ranging from 0.6 to 2.0 cows per stall (Bach et al., 2008). This study and another by Fregonesi et al. (2007) suggest a loss of 1.14 to 1.25 lbs (0.52 to 0.57 kg) of milk per increase of 0.1 cows per stall, equating to a 5.7 to 6.3 lbs (2.6 to 2.9 kg) reduction in milk per cow per day from 1 to 1.5 cows per stall.
• Overstocking is associated with elevated somatic cell counts (Krawczel and Grant, 2009) most likely due to an increase bedding bacteria. 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.

Increasing the number of cows in a pen when the structure of the pen does not change has negative impacts that have been less well investigated by peer-reviewed research. However, they still have an impact on the cow and management system. They are:

• Decreased water access, alley area, and an increase in manure. For a 200-stall head-to-head 2-row men moving from 1 to 1.5 stalls per cow:
  3. Accessible water trough perimeter is reduced from ~3.6 to 2.4 inches (~9.1 to 6.1 cm) per cow – less than the target of 3 to 4 inches (7.6 to 10.2 cm) per cow, making a reduction in water intake and competition around the waterers likely.
  4. Alley area is reduced from 60 to 40 sq ft (5.6 to 3.7 sq m) per cow. More cows mean more manure, but less area per cow in the alleys, leading to deeper manure and more leg and udder contamination, negatively impacting udder health.
  5. The manure handling system must deal with the increased manure in the pens, resulting in negative impacts on sand reclamation systems which struggle to handle the increase in organic solids.
• More cows per pen lead to longer milking times in conventional parlor herds and increased fetch rates in herds with automated milking systems (AMS). 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 robot 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 cu ft (71.5 cu m) per cow, which is reduced to 1,684 cu ft (46 cu m) 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 are most likely not equipped to deal with the results from adding more milking cows – Each of the cows that are 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 facilities that likely did not change to accommodate the added throughput of animals.

In summary, the negative impacts of overstocking creep up over months to years. Lameness rates increase because of the lost resting time, milk per cow goes down ~6 lbs (~3 kg) or more per cow per day, milk fat and protein is reduced, and SCC increases. Poor fresh cow performance manifests in climbing turnover and death rates and reduced peak yields, and fertility losses lead to a shortage of replacements. It may take 2 years to realize the full impact of the decision to overstock, but these are the signs of overstocking we repeatedly see over time.

References:

• Bach, A. et al.(2008) ‘Associations Between Nondietary Factors and Dairy Herd Performance’, Journal of Dairy Science. Elsevier, 91(8), pp. 3259–3267. doi: 10.3168/jds.2008-1030.
• Caraviello, D. Z. et al.(2010) ‘Analysis of Reproductive Performance of Lactating Cows on Large Dairy Farms Using Machine Learning Algorithms’, Journal of Dairy Science, 89(12), pp. 4703–4722. doi: 10.3168/jds.s0022-0302(06)72521-8.
• DeVries, T. J. (2019) ‘Feeding Behavior, Feed Space, and Bunk Design and Management for Adult Dairy Cattle’, Veterinary Clinics of North America – Food Animal Practice. Elsevier Inc, 35(1), pp. 61–76. doi: 10.1016/j.cvfa.2018.10.003.
• Fregonesi, J. A., Tucker, C. B. and Weary, D. M. (2007) ‘Overstocking Reduces Lying Time in Dairy Cows’, Journal of Dairy Science. Elsevier, 90(7), pp. 3349–3354. doi: 10.3168/jds.2006-794.
• Huzzey, J. M. M. et al.(2012) ‘The effects of overstocking Holstein dairy cattle during the dry period on cortisol secretion and energy metabolism’, Journal of Dairy Science. Elsevier, 95(8), pp. 4421–4433. doi: 10.3168/jds.2011-5037.
• Leonardi, C. and Armentano, L. E. (2003) ‘Effect of quantity, quality, and length of alfalfa hay on selective consumption by dairy cows’, Journal of Dairy Science. Elsevier, 86(2), pp. 557–564. doi: 10.3168/jds.S0022-0302(03)73634-0.
• Miller-Cushon, E. K. and DeVries, T. J. (2017) ‘Short communication: Associations between feed push-up frequency, feeding and lying behavior, and milk yield and composition of dairy cows’, Journal of Dairy Science. American Dairy Science Association, 100(3), pp. 2213–2218. doi: 10.3168/jds.2016-12004.
• Morabito, E. et al.(2017) ‘Effects of changing freestall area on lameness, lying time, and leg injuries on dairy farms in Alberta, Canada’, Journal of Dairy Science. American Dairy Science Association, 100(8), pp. 6516–6526. doi: 10.3168/jds.2016-12467.
• Omontese, B. O., Bisinotto, R. S. and Cramer, G. (2020) ‘Evaluating the association between early-lactation lying behavior and hoof lesion development in lactating Jersey cows’, Journal of Dairy Science. American Dairy Science Association, 103(11), pp. 10494–10505. doi: 10.3168/jds.2020-18254.
• Proudfoot, K. L. et al.(2010) ‘Behavior during transition differs for cows diagnosed with claw horn lesions in mid lactation’, Journal of Dairy Science. Elsevier, 93(9), pp. 3970–3978. doi: 10.3168/jds.2009-2767.
• Schefers, J. M. et al.(2010) ‘Management practices associated with conception rate and service rate of lactating Holstein cows in large, commercial dairy herds’, Journal of Dairy Science. Elsevier, 93(4), pp. 1459–1467. doi: 10.3168/jds.2009-2015.
• Sepúlveda-Varas, P. et al.(2018) ‘Claw horn lesions in mid-lactation primiparous dairy cows under pasture-based systems: Association with behavioral and metabolic changes around calving’, Journal of Dairy Science. American Dairy Science Association, 101(10), pp. 9439–9450. doi: 10.3168/jds.2018-14674.
• Tucker, C. B. et al.(2021) ‘Invited review: Lying time and the welfare of dairy cows’, Journal of Dairy Science, 104(1), pp. 20–46. doi: 10.3168/jds.2019-18074.
• Woolpert, M. E. et al.(2017) ‘Management practices, physically effective fiber, and ether extract are related to bulk tank milk de novo fatty acid concentration on Holstein dairy farms’, Journal of Dairy Science. American Dairy Science Association, 100(6), pp. 5097–5106. doi: 10.3168/jds.2016-12046.