Overstocking and Dairy Cows: Lessons from a 2012 Review

What Counts as Overstocking on a Dairy Farm?

A pen is overstocked when it holds more cows than it has lying stalls or places at the feed bunk, and researchers express the ratio as a percentage (Huzzey et al., 2012a). At 100% there is one stall or one headlock per cow. At 150% there are 1.5 cows for every stall, and at 200% two cows share each one (Huzzey et al., 2012a; Huzzey et al., 2012b).

Feed space is also measured as a length of feed barrier per cow. The 2012 review by Julie Huzzey, Marina von Keyserlingk and Thomas Overton took its benchmark from Canada’s dairy code of the time. That code, the review said, suggested one lying stall for every lactating cow and 60 cm (24 in) of linear bunk space (Huzzey et al., 2012a). The review’s own closing advice repeats both numbers as the aim for every producer (Huzzey et al., 2012a).

Overstocking rarely begins as a plan: the review names busy calving months, surplus replacement heifers and herds that grow before their barns do (Huzzey et al., 2012a). Crowding can also pay on paper, as a 2016 US simulation put the most profitable stall stocking at 120% on average, while calling the performance evidence weak (De Vries et al., 2016).

How common crowded pens are

The most recent national US figures come from the USDA’s Dairy 2007 study, published in 2010: 57.1% of freestall operations gave cows less than 60 cm (24 in) of bunk space, and 30.4% housed 1.10 or more cows per stall (USDA APHIS VS CEAH, 2010).

Newer data are regional. In 78 Canadian herds assessed in 2014–2018, mostly in Ontario, 41% of far-off dry pens and about half of close-up and fresh-cow pens were overstocked, although the visits were not a national sample (Couto Serrenho et al., 2022).

How Does Crowding Change Feeding and Lying?

Crowding changes when cows eat and how long they rest (Huzzey et al., 2012a). In one feeding study cited by the review, cows fed more when they had 1.0 m of bunk each than when they had 0.5 m (Huzzey et al., 2012a). The gap was widest in the 90 minutes after fresh feed arrived (Huzzey et al., 2012a).

That window matters because fresh feed is then at its best and cows are keen to reach it (Huzzey et al., 2012a). In a follow-up study, crowding at the bunk cut the share of cows feeding after delivery, whether the barrier was a post-rail or headlocks. The cows did not catch up overnight. They spent longer standing idle instead, apparently waiting for a place (Huzzey et al., 2012a).

Less feeding time does not always mean less feed, as crowded cows eat faster, and in one study multiparous cows crowded before calving tended to eat nearly 2 kg less dry matter a day (Huzzey et al., 2012a).

Lost lying time

Stalls matter even more to cows than feed places, and when both are scarce cows give up feeding time to lie down (Huzzey et al., 2012a). In one experiment, groups of 12 cows were given between 12 and 8 stalls, a range of 100% to 150% stocking (Fregonesi et al., 2007). The review reports that at 150% cows lay down 1.7 hours a day less. They also lay down 13 minutes sooner after milking, as if racing for a free stall (Huzzey et al., 2012a).

The losses were greatest at the usual peak resting times, overnight and around midday (Huzzey et al., 2012a). Time not spent lying is spent standing, often on concrete alleys. The review flagged long standing on hard floors as a known risk factor for lameness, although it did not measure lameness itself (Huzzey et al., 2012a).

What Did the Cornell Experiment Show About Stress and Metabolism?

The Cornell experiment showed that crowding can shift energy metabolism, though the shifts were small and the cortisol results mixed (Huzzey et al., 2012a; Huzzey et al., 2012b). Before it, very little work had looked at physiology. Studies from the 1970s had found a stronger cortisol response to an ACTH challenge in cows regrouped into pens with 25 cm of bunk per cow and one stall for every two cows (Huzzey et al., 2012a).

The trial used 40 pregnant, non-lactating Holstein cows, 16 heifers and 24 older cows, in groups of 10 at Cornell University (Huzzey et al., 2012a). Each group spent 14 days at 100% stocking and 14 days at 200%, with two cows per stall and half the bunk space, ending each period with a glucose tolerance test and an ACTH challenge (Huzzey et al., 2012a; Huzzey et al., 2012b).

When crowded, cows ate about 1 kg more dry matter a day, 15.9 against 14.9 kg, without spending longer at the bunk, so they ate faster (Huzzey et al., 2012b). Plasma NEFA and glucose were nonetheless higher, which the authors thought might reflect greater mobilisation of body reserves (Huzzey et al., 2012a; Huzzey et al., 2012b). In the glucose tolerance test, glucose cleared slightly more slowly and the insulin response was smaller (Huzzey et al., 2012b).

Cortisol gave a weaker signal. The adrenal response to ACTH did not change, and the faecal cortisol metabolite only tended to be higher during crowding (Huzzey et al., 2012b). The authors called their results the first evidence that overstocking can alter energy metabolism. They added that more work was needed before long-term effects could be judged (Huzzey et al., 2012a).

What Do Welfare Codes Require Today?

Most current codes and schemes now expect at least one stall or cubicle per cow, though only some put a number on feed space (National Farm Animal Care Council, 2023; EFSA Panel on Animal Health and Animal Welfare (AHAW) et al., 2023; Department for Environment, Food & Rural Affairs, 2003; Red Tractor, 2025; RSPCA, 2026). The main US programme sets no numeric ratio at all (National Dairy FARM Program, 2024).

Canada and the United States

Canada’s Code of Practice for the Care and Handling of Dairy Cattle has been in force since 1 April 2024. It caps free-stall density at 1.2 cows per stall (National Farm Animal Care Council, 2023). From 1 April 2027 density must not normally exceed 1.1 cows per stall, and from 1 April 2031 one cow per stall (National Farm Animal Care Council, 2023). Code requirements are an industry standard that may be enforceable under legislation, not a statute (National Farm Animal Care Council, 2023).

The code recommends 0.9 cows per stall and at least 60 cm of bunk per lactating cow and 76 cm per transition cow (National Farm Animal Care Council, 2023). In the United States, the National Dairy FARM Program asks only that animals have access to sufficient feed (National Dairy FARM Program, 2024).

England, the UK schemes and the EU

In England, Defra’s cattle welfare code, issued in April 2003, recommends at least one cubicle per cow and about 5% more cubicles than cows (Department for Environment, Food & Rural Affairs, 2003). Where feed is not available at all times, all animals in a pen should be able to eat at once (Department for Environment, Food & Rural Affairs, 2003). Red Tractor’s February 2025 dairy standards require at least one cubicle per animal and feeding space suited to the feeding system (Red Tractor, 2025).

RSPCA Assured goes further. Its April 2026 standards require at least 5% more cubicles than cows and strongly recommend 10–20% extra (RSPCA, 2026). Feed space must be 750 mm per head in buildings built from July 2020 and 600 mm in older ones, with 100 cm per transition cow and at least 15% spare cubicles for them (RSPCA, 2026).

The European Food Safety Authority’s 2023 scientific opinion concluded with 90% certainty that cows need at least one cubicle each, and with 66% certainty that they benefit from more cubicles than cows (EFSA Panel on Animal Health and Animal Welfare (AHAW) et al., 2023). It is advice to the European Commission, not law (EFSA Panel on Animal Health and Animal Welfare (AHAW) et al., 2023).

How Strong Is the Evidence?

The behavioural evidence is the strongest: later trials agree that crowding increases competition at the feed face (Huzzey et al., 2012a; Miltenburg et al., 2018; Fujiwara et al., 2019). The evidence on metabolism, health and milk yield is weaker and mixed, and the review’s new physiological data come from one crossover trial of 40 dry cows (Huzzey et al., 2012a; Huzzey et al., 2012b).

Some details in the review need care: it states that 43% of US freestall farms provided too few stalls (Huzzey et al., 2012a), while the USDA table shows about 43% stayed below one cow per stall (USDA APHIS VS CEAH, 2010).

What later trials found

Later trials have not reproduced the metabolic signal consistently. In a UK study of 48 dry cows, high stocking density increased competition at the feed face but did not affect stress responses, energy metabolism or later milk yield (Fujiwara et al., 2019). Close-up cows given 120% stall stocking and 45 cm of bunk lay 12.8 hours a day, against 14.8 hours at 80% and 90 cm (Miltenburg et al., 2018). Yet most of the expected harm to metabolic and immune markers did not appear (Miltenburg et al., 2018).

Counting cows against stalls and centimetres of feed face is simple arithmetic, yet it decides how much rest the lowest-ranking cow in the pen can get.

Behaviour is therefore the more direct measure. EFSA treats lying for less than 10–12 hours a day in cubicle housing as a sign of high welfare risk, to be read in context (EFSA Panel on Animal Health and Animal Welfare (AHAW) et al., 2023). By that yardstick, the lost lying time is the best-supported cost of overstocking, while effects on disease remain uncertain (Huzzey et al., 2012a; EFSA Panel on Animal Health and Animal Welfare (AHAW) et al., 2023; Miltenburg et al., 2018).

Frequently asked questions

Rhys Penwarden — author

Rhys Penwarden is an Animal Science graduate with postgraduate training in Livestock Production. His previous work focused on preparing technical materials on feeding and herd management for dairy and beef farming operations. At Cabdirect, Rhys covers dairy an...