Cover crop variety and post-pasture grazing cattle
What's the impact on ruminal microbiome composition?
By Mitchell VanderWal
Farmers and ranchers are often searching for more sustainable practices that will allow them to feed cows more efficiently and as cheap as possible. Maximizing profitability within a cow-calf operation requires that a least-cost winter feeding system be in place. As mid to late gestation nutrition requirements are at their lowest in the beef cow’s cycle, simply providing the cow with a forage source, such as crop residue or cover crops, that maintains the activity of rumen microbial communities is necessary for maintenance.
Because crop residues are a low-quality feed, they are only suitable as a large portion of the diet for mature beef cows (Krause 2013). In the U.S. Midwest region, many producers will turn cows onto corn stalks or small grain crop residue in the fall after pasture grass has lost its nutritional value. Cereal grain straws in particular have been extensively studied as a low-cost winter feedstuff for beef cows. Additionally, with the expansion in ethanol production, corn acreage has dramatically increased. Thus, corn stalk crop residues have been used primarily as a feed source for beef cattle in winter grazing systems (Lardner 2010).
Many producers have looked at planting alternative crops into their post-harvest residue, not only to boost soil health, but also to provide cows with an alternative nutrient source. While it is an additional step compared to simply allowing cattle to graze crop residue, it can have tremendous long-term benefits. For instance, cover crops have become a popular grazing method for mature beef cows during their second trimester of gestation. Plants such as clover, turnips, radishes, rapeseed, barley, triticale, oats, peas and many other varieties are popular to plant as a strategy to improve soil health.
For instance, corn yields have been shown to increase 3.2% after being planted into cover crop ground, whereas soybean yields have been shown to increase 4.6% (Barrera, L. 2014). Indeed, not only does planting cover crops assist with erosion, soil nutrient and water retention, organic matter residue and nitrogen recycling, it is a beneficial feed for livestock to consume. A strong correlation has been shown between increasing crop yields and net returns on calves produced, with positive returns ranging from $17 to $122 coming off of cover crop grazing. Therefore, a better understanding of how cover crops can be utilized by ruminant livestock is critical to further improve animal health and efficiency of nutrient utilization.
The rumen is responsible for the ability of ruminants to provide human edible nutritious foods derived from crops found on marginally productive land, without having to compete with humans for food crop production. Therefore, a better understanding of how the rumen microbiome digests feed in ruminant livestock can lead to further improvements in performance, boosting production to its utmost potential.
Knowing how the feeds being consumed are metabolized in the rumen can assist in increasing grazing and digestive efficiency in livestock production. The rumen harbors a complex anaerobic microbial ecosystem composed of protozoa, methanogenic archaea, bacteria, fungi and phages. Rumen microorganisms work in synergy to breakdown plant carbohydrates from feed into end products that provide the host with the energy it needs.
Materials and methodTo test this, batch cultures of rumen fluid were used to test cover crop varieties (rapeseed, radish, and turnip). After seven days in culture, samples were collected for DNA extraction. Bacterial composition was determined by sequencing the 16S rRNA gene. Using an approach based on Operational Taxonomic Units (OTUs), the most abundant bacterial species were identified.
ResultsThe results of the experiment showed differences in the average abundance of the bacterial populations. Figure 2 shows the top five most closely related bacterial species to species already known through DNA sequencing. Each column represents a different crop sample, that contains subsequent strains found in each rumen sample after testing: A Day 0 sample, negative control, each of the three radish, rapeseed, and turnip samples.
Each of the three cover crop varieties were averaged to find the abundance on average for each bacteria strain. Because the species sequenced from this study’s samples were identified using DNA sequencing and not from growth of isolates, candidate bacterial species or strains were assigned names MVW1 through MVW5.
Figure 3 shows the relative abundance (%) of each of the most abundant rumen bacterial species in relation to the specific cover crop used in their respective rumen fluid cultures. Those results demonstrated that while OTU MVW5 was found most prevalent in the Day 0 culture during grass grazing, it was of much less abundance with cover crops. This suggests that a transition to cover crops could result in a major change in bacterial composition.
Varying differences in abundance of OTU’s with cover crops showed that OTU’s may differ amongst the crops, and could be potentially specific. Activity in the microbiome of the rumen is noted to change when altering diets are offered, suggesting better utilization of specific crops by certain bacteria.
DiscussionThe rumen’s ability to utilize many different types of nutrients can be essential for increasing efficiency, gain and health. Increased activity in the rumen can lead to improved digestibility capacity; in general, a more diverse rumen microbial environment allows a broader capacity to provide energy for the host.
The focus for this specific project was to discover what rumen bacteria were most prevalent in the rumen (D0-inoculum), and how they behaved when a transition from grass to cover crops happened in an artificial environment (D7 cultures). However, the basic understanding of the results and implications of this project could be expanded upon in several ways. Performing further research regarding how the rumen microbiome behaves in varying diets could suggest more about the vast array of rumen bacteria. While it can be quite feasible to perform that research in an artificial rumen environment like this project, testing it in the field could be even more insightful. Furthermore, educating producers on the importance of data analysis could be tremendously beneficial when applying results into practical applications.
Many producers already utilize cover crops for boosting soil health, but there is a slight uncertainty of what the cover crop varieties can do to their ruminant livestock’s digestive systems. The results from this research project provide insight into what producers can expect for an alteration in their livestock animals’ rumen to best prepare for changes in diets. When talking about the option of grazing cows in the third trimester on cover crops – brassicas specifically – a few key points can be shared. Cows transitioning from grass to cover crops will utilize a different variety of bacteria within their rumen. Second, ensuring that cows are monitored for potential bloat risks or other digestive disorders is important. While much of the bacteria assisting in the breakdown of cover crops within the rumen seem beneficial, some strains may increase the risk of acidosis.
In southern U.S states, grazing cattle on wheat pasture is a very popular method of winter grazing prior to sending feeder cattle to the feedyard. Exploring the other possibilities of grazing varying kinds of forages, such as cover crops, could open the door to grazing stocker cattle on potentially cheaper and more beneficial crops from a performance standpoint. Researching the effect of stocker steers transitioning from weaning to grazing cover crops and its effect on feedlot and harvest performance could be an advantageous approach to a prominent livestock sector of the southern United States.
Results from this study indicated that some strains of bacterial species are heavily favored in the utilization of cover crops as a main nutritional source, demonstrating that prevalence of bacteria not commonly found in the beef rumen grass diet can be found of a much high abundance in cover crop samples. Suggesting that a greater variety of cover crops planted into grazing areas may allow a higher efficiency and production output potential in beef cattle. While this could be beneficial, higher abundance of specific bacterial species could also lead to bloat risks in beef cattle.
Future research is therefore needed to gain more information on the adaptation of rumen microbiomes to cover crop grazing in beef cattle. Such research may allow a greater understanding of potential advantages and drawbacks to exposing livestock to a diverse range of feedstuffs. In return, providing impactful and beneficial information back to the producer.
References available upon request.
VanderWal is a producer advisor with AgSpire.
