Difference Between White Clover And Alfalform
Results and Discussion The annual yield of different mixtures and or pure grasses differed over from one another over the course of the experiment. The three mixtures had a greater annual yield than pure grasses in all three seasons. White clover mixtures had similar yields in all seasons, however they were less productive than alfalfa mixtures in year 2 and 3. Also, grass yield in mixtures was greater than in pure cultures for all seasons despite its half-sowing density, and it was precisely greater in white clover mixtures than alfalfa mixtures. Legume yield increased from year 1 to 3 in all mixtures except for white clover-tall fescue, which retained a similar legume yield over the experiment. Legume proportion and yield was the greatest in alfalfa-tall fescue mixtures,
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followed by white clover-ryegrass and by white clover-tall-fescue mixtures. Legume population dynamics also differed between alfalfa and white clover. Indeed, alfalfa exhibited an overall decrease in plant density over the three years, but an increase in plant size. As a result, shoot density remained similar. On the other hand, white clover experienced drastic annual cycles where it almost reached extinction at the end of the season, but had a burst of growth in the following spring. This is consistent with what we know about root morphology of the two legumes; alfalfa will invest in a persistent taproot that will allow a more constant growth over multiple seasons while white clover grows many short-lived fine roots, making it less persistent. White clover also had a lower development when mixed with tall fescue than when mixed with perennial ryegrass. The nitrogen yield of legume was linked to the total legume yield and proportion in mixtures. Indeed, it was the greatest in alfalfa-tall fescue, followed by white clover-ryegrass and white clover-tall fescue in year 2 and 3, although it was similar in year 1. Moreover, nitrogen present in root nodules was almost all derived (>90% in all but two harvests) from the atmosphere, and this proportion was similar in alfalfa and white clover except for the first cut of year 1, where alfalfa required more time to get established. Moreover, there were noticeable differences in root yields and composition between white clover and alfalfa.
Although fine root biomass was similar in both species in the top 0-20 cm soil horizon, white clover had a smaller total root biomass than alfalfa. Also, white clover invests a greater proportion of its biomass into fine roots than alfalfa, which invests more in a large taproot. Taproots have a C/N ratio of about 25, which is larger than fine roots (~16), but a lower fiber and lignin concentration. As a result, the root system of alfalfa has a greater C/N ratio (19.3) and a lower lignin and fiber concentration than white clover (C/N ratio=13.9). When the C/N ratio is greater than 20, net immobilization of nitrogen occurs, meaning that it gets stored in soil microorganisms instead of being mobile in the soil and useable by plants. The lower this ratio is, the more nitrogen is available to meet the demands of decomposers, and the faster nitrogen is mineralized in the soil in a useable form. The lower C/N ratio and higher amount of N-rich fine roots suggest that white clover roots have a faster decomposition and turnover than alfalfa, and that more of this nitrogen gets mineralized in
soil. When measuring the nitrogen nutrition index (NNI) of grasses, the authors concluded that its value differed between grasses in pure cultures and in mixtures. Although it was below 1 (nitrogen deficiency) for all grasses, it had a higher value in mixed stands. The NNI also increased in mixed stands while it remained constant in pure stands. However, the rate of increase was different between the two mixtures; it increased less rapidly when mixed with alfalfa than white clover, probably because of the lower decomposition rate. Yet, the NNI in grasses mixed with alfalfa was greater in year 3 than when mixed with white clover, meaning that grasses with alfalfa have a more optimal growth regarding their nitrogen content. The authors also noticed that the NNI in mixed grasses started increasing before significant nitrogen transfer occurred. This could be explained by the niche complementarity of grasses and legumes. Indeed, grasses have a different root depth and characteristics, plant height and seedling emergence than legumes, therefore competition for light, nutrients and water is reduced in mixed stands relative to pure grass stands. Moreover, pure grass stands have a really dense, cespitose growth, therefore mixing grasses with legumes reduces grass density and allows for a greater amount of nitrogen to be available per grass plant. The low and constant NNI of pure stands can therefore both be explained by the absence of nitrogen transfer from a legume companion and of niche complementarity. Similarly to the NNI, the total nitrogen yield of mixed grasses was higher than that of pure grasses, but the nitrogen portion from the soil was lower. For white clover mixtures, both grasses had similar nitrogen yield. On the other hand, tall fescue with alfalfa had a total nitrogen yield similar to pure tall fescue in year 1, but became greater in year 2 and 3, where it reached a nitrogen yield similar to tall fescue-white clover. In addition, mixed grasses had a lower soil nitrogen proportion than pure grasses in year 2 and 3 with both alfalfa and white clover since a portion was derived from the legume companion. Yet, less nitrogen was derived from alfalfa (20-40%) than from white clover (60-70%), possibly because of the greater mineralization and decomposition rate of white clover roots. In year 1, tall fescue with alfalfa had a soil nitrogen proportion similar to pure tall fescue, and this could be, again, because of the lower decomposition rate of alfalfa roots. Indeed, alfalfa did not transfer any nitrogen in year 1. To calculate the net legume to grass nitrogen transfer in mixtures, the authors used both the difference and isotopic methods and they yielded similar patterns, yet different values. Indeed, they both showed that nitrogen transfer was greater and more rapid with white clover than alfalfa, although values were similar for both legumes in year 3. However, the isotopic method consistently yielded about 35% lower values the difference method. The authors explained this observation by the fact that adding a legume to a grass stand changes root distribution, possibly allowing an access to a greater nitrogen pool, and that this nitrogen input from the soil is included in the difference method, although not in the isotopic method which only includes atmospheric nitrogen input. The authors additionally observed that a reduced legume density was negatively and linearly correlated to the amount of nitrogen transferred to the grass companion. Based on their results, the authors concluded that differences in nitrogen transfer values and dynamics cannot only be accounted for by nitrogen fixation rates, legume proportions and root yield. Indeed, although alfalfa had greater values of these three factors, it transferred nitrogen at lower rates than white clover, resulting in grass companions having about half the nitrogen concentration from alfalfa than from white clover. They also calculated that, based on nitrogen fixation and transfer, white clover was five times more efficient at transferring fixed nitrogen than alfalfa.
Critique
There are a few issues with the experimental methods of this experiment. One of them is that the authors used unusual seeding rates for legumes in their mixtures. Indeed, alfalfa is recommended to be seeded at 9 kg ha-1 in mixtures, and fescue with alfalfa at 10 kg ha-1, however the authors used 12.5 kg ha-1 and 15 kg ha-1, respectively. Moreover, ryegrass should be 5.5 kg ha-1 and tall fescue 9 kg ha-1 when mixed with white clover, not 15 kg ha-1. By seeding with unusually high rates, the authors enhanced competition thus favored the species with a faster emergence and competitive advantage. Moreover, in the results, we could see that white clover had a lower percentage in mixtures than alfalfa. Yet, white clover was also seeded at a much lower rate, and if alfalfa was seeded at its appropriate rate, it probably would have had a lower proportion as well.
This last observation becomes important because, although the authors did not discuss the important of different legume proportions, it is crucial to consider it when feeding ruminant with these mixtures. Indeed, both alfalfa and white clover are considered high risk in causing bloating. A legume proportion of 50% or less reduces bloating but, in areas where bloating frequently occurs like in Quebec, the legume proportion is recommended to be below 25-30% (Reviewed by Majak et al., 2008). In figure 1, we can see that the alfalfa proportion is greater than 50% in all seasons. However, the authors did not test for significance between legume and grass proportions between the different treatments; they only tested for differences in total yield.
In addition, in all their experiments involving nitrogen transfer rates or the amount of nitrogen from soil and legume in grasses, the authors compared tall fescue-alfalfa with ryegrass-white clover, mixtures with different grass species. Indeed, by only labeling treatments with these different grass species, they assumed that both grasses were comparable in terms of nutrient uptake, growth patterns, dynamics and allelopathy effect on their legume companion. Yet, Moir et al. (2013) showed that the amount and rate of nutrient uptake differs between grasses. Also, there have been multiple studies that show that different grasses in mixtures with legumes vary in yield, and that they affect the net grass and legume proportion of the mixture (e.g. Sleugh et al., 2000; Albayrak & Türk, 2013; Berdahl et al., 2001). In addition, grass species have differential allelopathy effects on legumes, which could have reduce legume yield and performance in mixtures (Chung & Miller, 1995). All these examples show that in order to consistently compare the amount of nitrogen transferred and uptaken by a grass companion in mixtures, you need to use the same grass species.
Moreover, throughout their paper, the graphics were difficult to read and interpret; all symbols were very similar and they were sometimes sub-divided by grass species, sometimes by legume species. Therefore, we always had to carefully read the label for each new graph.
followed by white clover-ryegrass and by white clover-tall-fescue mixtures. Legume population dynamics also differed between alfalfa and white clover. Indeed, alfalfa exhibited an overall decrease in plant density over the three years, but an increase in plant size. As a result, shoot density remained similar. On the other hand, white clover experienced drastic annual cycles where it almost reached extinction at the end of the season, but had a burst of growth in the following spring. This is consistent with what we know about root morphology of the two legumes; alfalfa will invest in a persistent taproot that will allow a more constant growth over multiple seasons while white clover grows many short-lived fine roots, making it less persistent. White clover also had a lower development when mixed with tall fescue than when mixed with perennial ryegrass. The nitrogen yield of legume was linked to the total legume yield and proportion in mixtures. Indeed, it was the greatest in alfalfa-tall fescue, followed by white clover-ryegrass and white clover-tall fescue in year 2 and 3, although it was similar in year 1. Moreover, nitrogen present in root nodules was almost all derived (>90% in all but two harvests) from the atmosphere, and this proportion was similar in alfalfa and white clover except for the first cut of year 1, where alfalfa required more time to get established. Moreover, there were noticeable differences in root yields and composition between white clover and alfalfa.
Although fine root biomass was similar in both species in the top 0-20 cm soil horizon, white clover had a smaller total root biomass than alfalfa. Also, white clover invests a greater proportion of its biomass into fine roots than alfalfa, which invests more in a large taproot. Taproots have a C/N ratio of about 25, which is larger than fine roots (~16), but a lower fiber and lignin concentration. As a result, the root system of alfalfa has a greater C/N ratio (19.3) and a lower lignin and fiber concentration than white clover (C/N ratio=13.9). When the C/N ratio is greater than 20, net immobilization of nitrogen occurs, meaning that it gets stored in soil microorganisms instead of being mobile in the soil and useable by plants. The lower this ratio is, the more nitrogen is available to meet the demands of decomposers, and the faster nitrogen is mineralized in the soil in a useable form. The lower C/N ratio and higher amount of N-rich fine roots suggest that white clover roots have a faster decomposition and turnover than alfalfa, and that more of this nitrogen gets mineralized in
soil. When measuring the nitrogen nutrition index (NNI) of grasses, the authors concluded that its value differed between grasses in pure cultures and in mixtures. Although it was below 1 (nitrogen deficiency) for all grasses, it had a higher value in mixed stands. The NNI also increased in mixed stands while it remained constant in pure stands. However, the rate of increase was different between the two mixtures; it increased less rapidly when mixed with alfalfa than white clover, probably because of the lower decomposition rate. Yet, the NNI in grasses mixed with alfalfa was greater in year 3 than when mixed with white clover, meaning that grasses with alfalfa have a more optimal growth regarding their nitrogen content. The authors also noticed that the NNI in mixed grasses started increasing before significant nitrogen transfer occurred. This could be explained by the niche complementarity of grasses and legumes. Indeed, grasses have a different root depth and characteristics, plant height and seedling emergence than legumes, therefore competition for light, nutrients and water is reduced in mixed stands relative to pure grass stands. Moreover, pure grass stands have a really dense, cespitose growth, therefore mixing grasses with legumes reduces grass density and allows for a greater amount of nitrogen to be available per grass plant. The low and constant NNI of pure stands can therefore both be explained by the absence of nitrogen transfer from a legume companion and of niche complementarity. Similarly to the NNI, the total nitrogen yield of mixed grasses was higher than that of pure grasses, but the nitrogen portion from the soil was lower. For white clover mixtures, both grasses had similar nitrogen yield. On the other hand, tall fescue with alfalfa had a total nitrogen yield similar to pure tall fescue in year 1, but became greater in year 2 and 3, where it reached a nitrogen yield similar to tall fescue-white clover. In addition, mixed grasses had a lower soil nitrogen proportion than pure grasses in year 2 and 3 with both alfalfa and white clover since a portion was derived from the legume companion. Yet, less nitrogen was derived from alfalfa (20-40%) than from white clover (60-70%), possibly because of the greater mineralization and decomposition rate of white clover roots. In year 1, tall fescue with alfalfa had a soil nitrogen proportion similar to pure tall fescue, and this could be, again, because of the lower decomposition rate of alfalfa roots. Indeed, alfalfa did not transfer any nitrogen in year 1. To calculate the net legume to grass nitrogen transfer in mixtures, the authors used both the difference and isotopic methods and they yielded similar patterns, yet different values. Indeed, they both showed that nitrogen transfer was greater and more rapid with white clover than alfalfa, although values were similar for both legumes in year 3. However, the isotopic method consistently yielded about 35% lower values the difference method. The authors explained this observation by the fact that adding a legume to a grass stand changes root distribution, possibly allowing an access to a greater nitrogen pool, and that this nitrogen input from the soil is included in the difference method, although not in the isotopic method which only includes atmospheric nitrogen input. The authors additionally observed that a reduced legume density was negatively and linearly correlated to the amount of nitrogen transferred to the grass companion. Based on their results, the authors concluded that differences in nitrogen transfer values and dynamics cannot only be accounted for by nitrogen fixation rates, legume proportions and root yield. Indeed, although alfalfa had greater values of these three factors, it transferred nitrogen at lower rates than white clover, resulting in grass companions having about half the nitrogen concentration from alfalfa than from white clover. They also calculated that, based on nitrogen fixation and transfer, white clover was five times more efficient at transferring fixed nitrogen than alfalfa.
Critique
There are a few issues with the experimental methods of this experiment. One of them is that the authors used unusual seeding rates for legumes in their mixtures. Indeed, alfalfa is recommended to be seeded at 9 kg ha-1 in mixtures, and fescue with alfalfa at 10 kg ha-1, however the authors used 12.5 kg ha-1 and 15 kg ha-1, respectively. Moreover, ryegrass should be 5.5 kg ha-1 and tall fescue 9 kg ha-1 when mixed with white clover, not 15 kg ha-1. By seeding with unusually high rates, the authors enhanced competition thus favored the species with a faster emergence and competitive advantage. Moreover, in the results, we could see that white clover had a lower percentage in mixtures than alfalfa. Yet, white clover was also seeded at a much lower rate, and if alfalfa was seeded at its appropriate rate, it probably would have had a lower proportion as well.
This last observation becomes important because, although the authors did not discuss the important of different legume proportions, it is crucial to consider it when feeding ruminant with these mixtures. Indeed, both alfalfa and white clover are considered high risk in causing bloating. A legume proportion of 50% or less reduces bloating but, in areas where bloating frequently occurs like in Quebec, the legume proportion is recommended to be below 25-30% (Reviewed by Majak et al., 2008). In figure 1, we can see that the alfalfa proportion is greater than 50% in all seasons. However, the authors did not test for significance between legume and grass proportions between the different treatments; they only tested for differences in total yield.
In addition, in all their experiments involving nitrogen transfer rates or the amount of nitrogen from soil and legume in grasses, the authors compared tall fescue-alfalfa with ryegrass-white clover, mixtures with different grass species. Indeed, by only labeling treatments with these different grass species, they assumed that both grasses were comparable in terms of nutrient uptake, growth patterns, dynamics and allelopathy effect on their legume companion. Yet, Moir et al. (2013) showed that the amount and rate of nutrient uptake differs between grasses. Also, there have been multiple studies that show that different grasses in mixtures with legumes vary in yield, and that they affect the net grass and legume proportion of the mixture (e.g. Sleugh et al., 2000; Albayrak & Türk, 2013; Berdahl et al., 2001). In addition, grass species have differential allelopathy effects on legumes, which could have reduce legume yield and performance in mixtures (Chung & Miller, 1995). All these examples show that in order to consistently compare the amount of nitrogen transferred and uptaken by a grass companion in mixtures, you need to use the same grass species.
Moreover, throughout their paper, the graphics were difficult to read and interpret; all symbols were very similar and they were sometimes sub-divided by grass species, sometimes by legume species. Therefore, we always had to carefully read the label for each new graph.