Rich Smith and Nick Warren University of New Hampshire, Department of Natural Resources and the Environment
Legumes really are extraordinary plants. Consider the fact that their family contains well over 16,000 species, members of which can be found growing in nearly every habitat on earth and range in size from the tiny clovers you might find growing in your lawn to some of the tallest trees in a tropical rainforest. Even more impressive is the symbiosis that has developed between most legume species and the specialized soil bacteria, rhizobia.
This mutualistic relationship involves the creation of nodules in the legume’s root system that house, feed, and protect the bacteria. In exchange the rhizobia living in the nodules convert the nitrogen gas in the air into a form that the legume can use—a process known as nitrogen fixation. Because of this mutually beneficial relationship with rhizobia, legumes are essentially able to “create” their own fertilizer. This means that most legumes can grow well in soils that are relatively poor in nitrogen. It also means that the roots, stems, leaves, and reproductive structures of legumes contain high quantities of nitrogen-rich compounds, such as protein, and over time, their growth can increase the amount of nitrogen in the soil that is available to other plants growing nearby. It is perhaps no surprise then that many of our most important agricultural crops are legumes. These run the gamut from food crops such as peanuts, beans, and peas; feed grains such as soybeans; and cover crops such as hairy vetch, whose primary purpose is to act as nitrogen-rich green manure. And of course, many of our most important forage crops are legumes.
Forage legumes can be critical to the success of organic dairy operations. Not only are they a palatable and nutritious component of most dairy diets, as explained in the recent NODPA article by Andre Brito and colleagues (Brito et al., 2020), forage legumes can also be key to maximizing overall forage yield in pastures and hayfields, optimizing forage quality and energy and protein levels to improve milk yield and milk fat and protein components, and to successfully implementing high-forage diets to capitalize on “grass-fed” and other specialty milk markets. Unfortunately, forage legumes can be difficult to establish and once established to keep around. This is particularly the case in pastures and hayfields with adequate soil nitrogen and when legumes are grown in mixtures with other competitive forage species such as grasses. In addition, pasture management can be considered a stress from a forage’s point of view, and frequent defoliation and grazing can reduce legume vigor and overwinter survival. And because forage legumes are often harvested or grazed before they can reseed, they tend not to regenerate naturally. Consequently, many dairy farmers in the Northeast and elsewhere report significant challenges in maintaining their desired proportions of legumes in their pastures or hayfield swards.
At the University of New Hampshire (UNH), we are conducting research to better understand how forage harvest practices influence the productivity and persistence of different forage legumes. This work is part of a USDA NIFA grant to evaluate the productivity and quality of grass-legume mixtures. While the complementary effects of pairing grasses and legumes has been well established—grass-legume mixtures often have higher total productivity and nutritional quality than monocultures of either grasses or legumes alone—we’re especially interested in learning how harvest management affects these mixtures over time. Specifically, we are interested in how the frequency and height of cutting influence the performance and persistence of the legume and grass components of the mixture. Currently, we are focusing on just four species of perennial forage legumes: alfalfa, white clover, red clover, and birdsfoot trefoil; however, we will likely be expanding that list soon to include additional species. Why? Because each species of forage legume will likely differ in their requirements for successful establishment; response to harvesting; nutritional attributes; tolerance to local climate, soils, and pests; and competitive abilities. So, simply put, it’s good to have options when it comes to forages. Birdsfoot trefoil, for example, has several forage qualities that are complementary to alfalfa – low risk of bloat, higher tolerance to flooding, and production of condensed tannins. Some forage legumes may have advantages while others may have drawbacks; therefore, the more we know about the wide variety of forage legumes that are currently available, the more helpful we can be to dairy farmers looking to implement specific forage legumes into their dairy systems.
The results described below come from a field experiment that was established at the UNH Kingman Research Farm in the fall of 2018. The data come from the 2019 growing season. Interestingly, we had attempted to establish the same study at a different site the prior fall; however, by the subsequent spring it was clear that the legumes had failed to establish properly, likely due to the unfavorable weather we experienced during that period, illustrating just what a challenge establishment can be sometimes. Nevertheless, we have our first full growing season of data to share with you. Keep in mind that a single season of data cannot tell us too much about persistence over time. We expect that the data we will continue to collect this season and the next will be very illuminating in that regard. That said, we think these preliminary data are quite interesting and suggestive of some general trends associated with harvest practices that might be useful for farmers to investigate in their own systems. Here is what we did and what we are seeing.
This is a relatively large and complicated experiment to manage, with many plots, each featuring a specific perennial forage legume mixed with a perennial forage grass, with specific harvest schedules and harvest heights. This overhead photo shows the entire experimental area after a midsummer harvest (Figure 1A), while the close-up photo shows two individual plots side-by-side that are just about ready for harvest (Figure 1B). Plots received a compost application in the summer and then were seeded in early September of 2018. Each plot was planted with one of four legumes in combination with a grass. The proportion of seeds sown in each plot was 70% legume seed and 30% orchardgrass seed, and the specific cultivars of each legume were ‘406AP2’ (alfalfa), ‘Alice’ (white clover), ‘Freedom’ (red clover), and ‘Bruce’ (birdsfoot trefoil). The orchardgrass cultivar ‘Latar’ was used as the companion grass in all plots. No other amendments, fertilizers, or pesticides were applied. There was good germination and overwinter survival despite region-wide issues with winterkill. We initiated the cutting frequency and height treatments in the spring of 2019 using a plot-scale forage harvester that could be set to variable cutting heights (Figure 2). The 3X frequency treatment involved harvesting each plot a total of 3 times over the growing season, beginning June 3 and ending October 15. In this treatment there was a relatively long interval and recovery time between each harvest. In contrast, the 5X frequency treatment involved harvesting each plot a total of five times over the season, also beginning on June 3 and ending October 15, creating a shorter interval and recovery period (30 days) between cuts. In addition to harvest frequency, plots were also assigned to a cutting height treatment. At the time of harvest, plots were cut to a height of either 5 cm (2 inches) or 10 cm (4 inches). These two fundamental management decisions, cutting height and frequency, may be useful approaches to maintaining healthy legume populations, but are likely to affect each species differently. The data below showing the response of each of the legumes and orchardgrass to the cutting frequency and height treatments are means of five replicate plots receiving that specific combination of treatments.
So, what did we observe over the 2019 field season? First, overall, red clover was much more productive compared to white clover, birdsfoot trefoil, or alfalfa, which seemed to do particularly poorly (Figure 3). In fact, red clover often made up more than 50% of the total forage dry matter regardless of the cutting frequency or height treatment, while the proportion of the other legumes was always substantially less. Keep in mind, however, that we had a particularly hard winter in 2018/19 and reports across the region indicated that alfalfa stands were especially hard hit in some areas. Therefore, the low productivity of alfalfa in our study may be an anomaly. Similarly, we’ve had trouble establishing robust stands of birdsfoot trefoil in the past, so its growth in this study may not be indicative of its true potential elsewhere.
The second key takeaway from this first season of the study was that harvest frequency had a greater impact on total forage production (total amount of forage dry matter produced over the season) than did cutting height. Specifically, there was greater total forage yield in the 5X compared to the 3X treatment and this was apparent regardless of which legume was included in the mixture (Figure 3). Importantly, most of this difference in total dry matter production was due to the abundance of the orchardgrass component of the mixture, which was on average 40% greater in the 5X treatment. In other words, the orchardgrass was much more responsive to the cutting frequency than were any of the four legume species. 
Finally, we observed that the proportion of legumes in each mixture varied over the course of the season and the trends were species-specific. This was particularly apparent in the 5X cutting treatment (Figure 4). For example, the proportion of red clover was nearly 70% of the total dry matter at the first harvest, just over 40% at the fourth harvest, and a little higher than that at the fifth and final harvest of the season. In contrast, both alfalfa and birdsfoot trefoil made up just over 35% of the total dry matter at the first harvest but not more than 5% by the final harvest. White clover, on the other hand, exhibited an altogether different trend. White clover made up a meager 20% of the total dry matter at the first harvest; however, by the final harvest it had increased to well over 30%.
Many organic dairies rely on forage legumes for their success; however, legumes can be difficult to maintain in certain pasture and hayfield situations. Our preliminary data suggest that harvest intensity can be an important factor in managing legume populations. Keep in mind that these data come from a single year and a single site and therefore may not be reflective of patterns observed elsewhere. Also keep in mind that we utilized a single companion grass (orchard grass) and that for each of the legume species we investigated a single cultivar. Other legume cultivars or grass-legume mixtures may be expected to perform differently. We expect to learn much more over this coming growing season and into the future regarding how management intensity, cutting height, and other variables influence the dynamics and persistence of these and other perennial forage legume species. Stay tuned for future updates!
Brito, A.F., Lange, M.J., and Silva, L.H.P. 2020. The key role of forage legumes in organic dairy diets: effects on your bottom line. NODPA News. Available at /n/945/The-Key-Role-of-Forage-Legumes-in-Organic-Dairy-Diets-Effects-on-Your-Bottom-Line
Rich Smith, Ph.D., is an Associate Professor of Agricultural Ecology, Department of Natural Resources and the Environment, University of New Hampshire, Durham, NH 03824. He can be reached at: office phone (603) 862-2724; email: richard.smith@unh.edu
Nick Warren is a Research Scientist and Graduate Student, Department of Natural Resources and the Environment, University of New Hampshire, Durham, NH 03824. He can be reached via email at nicholas.warren@unh.edu
