The increasing cost of straw is the reason behind the increasing switch towards alternatives. However, according to the researchers, the microbiological characteristics and production of recycled manure solids bedding are poorly documented under on-farm conditions. This bedding could support the presence of pathogens and could influence cow and human health.
In this study, researchers compared the microbiota composition and diversity of the recycled manure solids bedding and straw bedding, before (unused) and after their use as bedding material for dairy lactating cows.
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Unused and used bedding from 27 recycled manure solids and 61 straw-bedded dairy farms in eastern Canada were collected and compared using 16S amplicon sequencing, bacterial counts, and Salmonella spp. and Listeria monocytogenes detection. A second objective was to investigate the presence of 2 zoonotic agents, Salmonella spp. and L. monocytogenes, in both bedding types.
Recycled manure solids bedding
Recycled manure solids bedding can be produced on the farm. Usually, a fraction of the liquid contained in the manure is extracted using a screw or roller press. The liquid fraction of the manure is commonly used as an organic fertiliser in fields. The remaining solid fraction can be used immediately as bedding, but it is often further processed using various maturation methods. During maturation, the material’s temperature should increase to reduce the total bacterial concentration. On larger farms, anaerobic digesters can be used.
According to the researchers, there is currently no legislation guiding the production or use of recycled manure solids in Canada. This situation has led to a large variation in the production methods of recycled manure solids. Some studies have identified various benefits associated with recycled manure solids, such as increased comfort, reduced hock lesions, and improved cleanliness of dairy cows. On the other hand, other studies have observed larger undesirable bacterial populations, such as populations of Klebsiella spp., Escherichia coli, Streptococcus spp., and Staphylococcus spp.
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Microbiota in unused bedding
The results of the study showed significantly higher bacterial counts in unused recycled manure solids than in unused straw, except for Klebsiella spp. The proportions were as follows: L. monocytogenes (30% vs 11% in straw), L. Salmonella spp. (11% vs 1.6% in straw). The microbiota diversity was higher in unused recycled manure solids than unused straw bedding. Staphylococcus spp., Streptococcus spp., and E. coli counts were estimated to be 2.6, 1.6, and 2.0 log 10 cfu/g higher, respectively, in unused recycled manure solids bedding compared to straw bedding.
Microbiota in used bedding
As opposed to bedding material before use (unused), most phyla were observed in similar proportions in both used recycled manure solids and used straw samples. Used recycled manure solids and straw microbiota were composed mostly of Proteobacteria (53% vs 46% in straw), Bacteroidetes (25% vs 22% in straw), Firmicutes (10% vs 20% in straw), and Actinobacteria (8% vs 10% in straw).
The researchers detected L. monocytogenes in 48% of used recycled manure solids samples compared to 20% of used straw samples. Salmonella spp. was detected significantly more frequently in used recycled manure solids (15%) than in used straw samples (2%).
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Used straw samples had significantly higher Streptococcus spp. counts (7.4 log 10 cfu/g) than the samples of recycled manure solids. No substantial differences between used recycled manure solids and used straw bedding for Staphylococcus spp. or for Klebsiella spp. counts. As opposed to the observation in unused bedding material comparisons, the microbiota diversity was not different between used RMS and used straw.
Evolution of microbiota with use
Microbiota in the samples originating from unused straw were compared with those of used straw to assess the evolution of microbiota during use. The same comparison was made for recycled manure solids bedding.
The relative abundance of Proteobacteria was greater in unused straw than in used straw bedding, but Actinobacteria and Firmicutes were associated more with used straw. However, the trend was different in recycled manure solids where the Firmicutes were abundant in unused recycled manure solids compared with used recycled manure solids samples, and the relative abundances of Proteobacteria and Bacteroidetes were greater in used than in unused recycled manure solids samples.
Greater microbiota diversity was found in used straw bedding samples compared to unused straw samples, whereas the diversity was lower in unused recycled manure solids compared to used recycled manure solids samples. In recycled manure solids, it was stated that microbiota composition could be affected by regional factors such as humidity, temperature, and feed composition. The microbiota in straw was related to the effects of differences in origin, soil, or climate.
Influence of method of preparation on recycled manure solids
Finally, the microbiota in the samples originating from the unused recycled manure solids samples produced using 2 different production systems was compared. One method involved separation followed by maturation in a heap and in the other, maturation was done in an open or enclosed container. The results showed that heap-matured recycled manure solids samples had lower bacterial counts than enclosed container-matured samples.
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The researchers highlighted the importance of maturation during the processing of recycled manure solids on microbiota composition. Maturation can increase the temperature for maximal pathogen destruction, microbial stabilisation, and moisture reduction.
According tot he researchers, enclosed and ventilated containers may be useful as they enable a more standardised control of environmental conditions and can thus provide a uniform thermophilic treatment and year-round production of bedding with consistent quality. They added that chemical conditioners, such as alkaline conditioners can be used to reduce bacterial counts in recycled manure to improve its quality.
Summary
The microbiota of recycled manure solids bedding differed from that of straw, and the method for producing the manure solids clearly affected its quality. In this study, heap production appeared to be a better option for producing manure solids as it resulted in a lower bacterial load, and Listeria monocytogenes and Salmonella spp. were less frequently isolated. However, the researchers suggested that this would have to be confirmed in a study designed specifically for comparing production methods, as several methods also exist.
Overall, this study clearly indicated that the microbial quality of recycled manure solids is inferior to straw bedding. According to the researchers, the manure solids bedding currently produced on farms in eastern Canada clearly constitutes a greater microbiological risk for dairy herds compared with straw bedding.
“In future, an experimental study design would be needed to confirm our results as this study used an observational cross-sectional study design,” they said.
