In the face of an increase in new mastitis infections, it is important to identify any environmental or management changes that have recently occurred. Part of this investigation may include assessment of bedding, particularly where analysis of first clinical mastitis event data and/or new high cell count data highlight increases in new infections during periods of housing. This article summarises the literature on bedding materials for dairy cows, with a focus on udder health.
Bedding systems
Before exploring bedding materials, it is important to understand how bedding is used. Dairy cow lying areas can be designed in two ways:
- Loose housing: a large bedded area where cows can choose where to lie. Ideally the lying area would be separate from the loafing area and feeding areas. The loafing area is defined as a non-lying, non-passageway, non-feeding area. Typically these farms use deep bedding with straw or woodchip, sand is used less commonly. Recycled manure solids (RMS) are not allowed to be used in loose housing (Department for Environment, Food and Rural Affairs (DEFRA), 2016).
- Cubicle systems: cows are guided to lie in individual spaces. Cubicle bedding systems are a more efficient use of space, and can be shallow or deep bedded:
- Shallow bedded: thin coverage of bedding material <10 cm, typically on top of a mattress to provide comfort. Shallow-bedded systems commonly use straw, sawdust or recycled manure solids. Shallow bedding on top of concrete does not provide acceptable cow comfort
- Deep bedded: deeper covering of bedding material ≥10 cm, usually on top of a concrete or hardcore base. Deep-bedded systems commonly use sand, sawdust or recycled manure solids.
Loose housing, shallow-bedded and deep-bedded cubicles provide different levels of cow comfort, and this can impact cow behaviour and lameness. For more information on cow comfort, readers are directed towards an article by Cook and Nordlund (2009). The present article focusses on the impact of bedding materials on udder health.
Bedding types
The most commonly used bedding materials are straw, sand, sawdust and recycled manure solids (see Figure 1). Other bedding types include recycled paper, rape straw, bracken, pea straw and miscanthus (Agriculture and Horticulture Development Board, 2018). A survey of UK producers in 2011 found that straw was the most commonly used bedding material, followed by sawdust/shavings then sand (Shipton et al, 2011). This survey was carried out prior to the introduction of recycled manure solids bedding to the UK.
Broadly, bedding is often split into inorganic (sand) or organic (originating from plant or plant material). Recycled manure solids is a relatively new organic bedding material, and is created by drying out slurry using a press or clamp system and mechanically recovering undigested fibre material. Concerns have been raised about the risk of using recycled manure solids, and transmission of pathogenic bacteria, for example Mycobacterium avium paratuberculosis (Leach et al, 2015), Salmonella and Listeria species (Jørgensen and Jensen, 2009). As a result, all farms using recycled manure solids are required to notify the Animal and Plant Health Agency (APHA) and Red Tractor, among other specific conditions (DEFRA, 2016). Between 2017 and 2021, 21 UK dairy farms notified the APHA that they were using recycled manure solids bedding (APHA, Personal Communication, October 2023), although it is expected that more farms are using this material.
Several studies have investigated different bedding materials, but the majority are observational, using a cohort of farms. In observational studies it is difficult to control for variation between farms, and any differences could be related to environment, for example ventilation, loafing area and scraping frequency. However, observational studies give a good indication of how bedding is managed on working farms, and can still give valuable results. In all studies comparing cohorts of farms there is large variation between farms, suggesting that some are managing bedding material well and some are not.
Importance of dry matter
Bacteria thrive in warm, moist conditions. In simple terms, the higher the dry matter content of bedding, the more capacity there is to absorb moisture from the environment.
Table 1 shows the typical dry matter content for the most commonly used bedding materials, according to published literature. Average dry matter content is highest in sand and sawdust bedding, slightly lower in straw and significantly lower in recycled manure solids. According to DEFRA (2016) recommendations, recycled manure solids bedding must reach at least 34% dry matter content before use. This level of dry matter is achievable using press or drum systems, and was the median dry matter content across a cohort of UK farms (Bradley et al, 2015). In some farms in the USA, high temperatures are used during preparation to increase the dry matter content. Composting and digestate is also used in some US farms, though this is not currently allowed under UK law.
Table 1: Dry matter content of fresh bedding material, as reported in the literature.
| Bedding type | Average dry matter content (%) | References |
|---|---|---|
| Fresh sand | 92–95 | Hogan et al, 1989; Bradley et al, 2015; Kristula et al, 2005; Robles et al, 2020 |
| Recycled sand | 89 | Kristula et al, 2005 |
| Sawdust | 85–93 | Bradley et al, 2015; Robles et al, 2020 |
| Straw | 64–88 | Hogan et al, 1989; Robles et al, 2020 |
| Recycled manure solids (RMS) | 25–38 | Bradley et al, 2015; Fournel et al, 2019; Robles et al, 2020 |
When assessing dry matter it is also important to consider bedding storage. Some bedding materials will dry out during storage, whereas others will absorb atmospheric moisture. It is essential that bedding materials are stored under cover. For recycled manure solids bedding, it is a requirement that bedding is used within 24 hours of manufacture to minimise absorption of moisture and bacterial growth (DEFRA, 2016), and therefore it is advised that this material is produced under cover.
During use, the dry matter of bedding will vary. A large bedding survey was carried out in 2014 and 2015 across a cohort of UK farms (Bradley et al, 2015). In this study, dry matter was unaltered in used sand bedding, reduced in sawdust, and increased in recycled manure solids (Bradley et al, 2015). It is important to highlight significant variation between farms. Dry matter content of bedding relies on how moisture is able to leave the building, so it is important to consider stocking density, slurry removal and ventilation (Leso et al, 2021).
Bacteriological analysis
From an udder health perspective, the ideal bedding material should limit bacterial growth. It should be low in bacterial numbers before use, and not show excessive growth during use. When investigating environmental mastitis, coliform counts are the most reliable indicator of udder health risk. Counts of greater than 1 000 000 cfu/ml have been associated with an increased risk of intramammary infection (Bramley and Neave, 1975; Hogan et al, 1989). In the study by Hogan et al (1989), there was a weak positive correlation between Gram-negative bedding bacterial count and clinical mastitis rate (r2=0.14), suggesting that bedding bacterial count accounts for 14% of the variation in clinical mastitis rate between different farms.
In the study by Bradley et al (2015), all samples of fresh bedding achieved coliform counts below the target of 1 000 000 cfu/g, although they were higher in fresh recycled manure solids bedding than sand or sawdust. Streptococcus and Staphylococcus species showed a similar pattern, with negligible levels in sand and sawdust, and higher levels in recycled manure solids. During use, bacterial counts were increased, although there was wide variation: coliform counts ranged between 3550 and 42 500 000 cfu/g. In used bedding, coliform counts were above the threshold of 1 000 000 cfu/g for 55% of recycled manure solids samples, 15% of sand and 9% of sawdust.
Studies published in other countries have found similar results, ie higher coliform counts in recycled manure solids bedding relative to sand (Godden et al, 2008; Rowbotham and Ruegg, 2016; Patel et al, 2019; Rowe et al, 2019; Robles et al, 2020). Within each bedding type there are differences: recycled sand has been shown to have higher coliform counts than fresh sand (Kristula et al, 2005; Rowbotham and Ruegg, 2016; Rowe et al, 2019); fresh recycled manure solids has lower bacterial counts than recycled manure solids that was digested, composted or matured in a heap (Godden et al, 2023).
When testing bedding materials for dry matter or bacterial counts, it is important that they are sent in a sealed bag, and travel chilled (less than 5°C) using ice bricks to minimise change during transit.
Cow cleanliness
Few studies have evaluated the impact of bedding material on cow cleanliness, but it is an important metric to consider as part of any mastitis investigation. Cow cleanliness should be assessed at the cow-level: in the environment (grazing or housed) and during milking. Indirect measures of cow cleanliness should also be included, such as inspection of milk filters, bactoscan or lactation new infection rates.
One study found that farms using recycled manure solids bedding had poorer cow cleanliness (Patel et al, 2019), although there was significant variation. Coliform counts from teat swabs were lower in herds bedding on sand relative to sawdust (Zdanowicz et al, 2004) or recycled manure solids (Rowbotham and Ruegg, 2016). However, both studies found higher counts of Streptococcus species on teats from sand-bedded cows. This is consistent with previous research showing that Streptococcus species survive better on sand relative to organic bedding (Sherwin et al, 2021).
Mastitis risk
Despite higher bacterial counts in organic bedding, and higher levels of coliform bacteria at the teat end, there is no clear association between bedding type and mastitis rate (Bradley et al, 2015; Rowe et al, 2019; Fréchette et al, 2021). Two of these studies found a numerically higher rate of mastitis on farms using recycled manure solids bedding, although they lacked sufficient power to detect a statistically significant difference (Bradley et al, 2015; Fréchette et al, 2021). Given that many other management factors influence mastitis risk, large sample sizes are required to detect significant associations in these observational studies.
One factor that is likely to impact mastitis risk is pre-milking teat disinfection. This has been shown to reduce bacterial count at the teat-end (Rowbotham and Ruegg, 2015), and is likely to impact mastitis rate through reduced rates of first infections in cows more than 30 days in milk (lactation origin). Pre-milking teat disinfection is recommended for all farms, but is mandatory for those using recycled manure solids (DEFRA, 2016).
Differences in mastitis aetiology have been observed. One study found 7x higher odds of mastitis caused by Klebsiella pneumoniae on recycled manure solids farms compared with those bedded on straw (Fréchette et al, 2021). This species is worthy of note, as treatment success is poorer than other environmental pathogens (Roberson, 2012). In contrast, this study found lower rates of S. aureus mastitis in recycled manure solids compared to straw bedding. It should be highlighted that the majority of recycled manure solids farms in this study were using composted or digestate recycled manure solids, which is not allowed in the UK.
Bulk milk
A small number of studies describe bulk milk quality on farms using different bedding materials. Bactoscan is a measure of the bacterial content of milk, and can be influenced by many factors such as cow and environmental cleanliness, mastitis detection, plant cleaning, and milk storage (Manning et al, 2021). Bulk milk somatic cell count (BMSCC) is a measure of mastitis detection and the prevalence of infected cows (particularly those with Gram-positive infections). The UK bedding survey found no clear association between bedding type and bulk milk bacterial counts or cell count (Bradley et al, 2015). However, use of pre-milking teat disinfection was associated with reduced pyschrotrophic bacteria on farms using recycled manure solids bedding. Another UK study found farms using straw cubicles had higher average bactoscan, relative to cubicles with other bedding materials (Shipton et al, 2011). Bulk milk somatic cell count was highest on farms with shavings and straw bedding, relative to farms using paper, sawdust or sand. A study from the USA found BMSCC was lowest in farms using inorganic bedding relative to organic non-manure bedding or recycled manure solids (Rowbotham and Ruegg, 2015). For farms using organic bedding, more regular bedding replacement was associated with lower BMSCC, suggesting better control of environmental infections. According to the Mastitis Control Plan, organic bedding material should be topped up daily, and sand bedding every other day.
Treatment of bedding materials
Several products are advocated as bedding additives, though the evidence base in this area is extremely weak. Dehydrated lime is the most commonly used bedding additive, and is often sprinkled on the back of shallow bedded cubicle systems, in the area where the udder would lie. Addition of lime has been shown to reduce bacterial counts in sawdust bedding, and subsequently on cows' teats (Fairchild et al, 1982; Hogan and Smith, 1997, Kristula et al, 2008). With high use, lime has the potential to cause skin irritation, and so teat condition should be monitored. Lime can be added to shallow straw and sawdust bedding (typically as a mattress dressing), but should be avoided with sand as it will combine to form concrete. Lime is not commonly used for deep bedded systems, for example those using recycled manure solids.
Aside from dehydrated lime, several chemical and biological agents are marketed, although there is limited peer reviewed evidence to justify their use. In the author's experience, some farmers are advised to spray beds with disinfectant chemical. There is no evidence to support this practice, many disinfectants are rapidly inactivated in the presence of organic material, and liquid preparations will add moisture.
Conclusions
Across the studies discussed in this article there is high variation in dry matter content, bacterial counts and mastitis rates between farms. These studies show that all bedding material can be managed well, and no single bedding type is superior. Bedding needs to be good quality before use, it needs to be used in sufficient quantities, and in a building with adequate space and ventilation.
KEY POINTS
- Bedding quality and management should be considered as part of mastitis investigations.
- At a herd level, bedding coliform counts of >1 000 000 cfu/g are associated with an increased risk of mastitis.
- Inorganic bedding materials are generally associated with lower bacterial counts than organic bedding – although large variation exists between farms.
- All bedding materials can be managed well.
- It is important to consider dry matter as part of bedding investigations.
- There is an association between recycled manure solids bedding and Klebsiella mastitis.
- There is limited evidence to support bedding ‘additives’.