Do High-Magnesium Cool-Season Grasses Contemplate Grass Tetany Risk? ()
1. Introduction
Optimization of mineral nutrition is important to maximize crop productivity. Mineral nutrients especially Mg protect crops against various abiotic stresses, including heat [1], high light radiation [2], drought, secondary salinity, soil acidity, and aluminum (Al) toxicity [3]. Optimum Mg nutrition is also essential for the healthy growth of animals but is a challenge, especially for grazing livestock. Adequate amounts of available Mg are required in soils and in the grass to fulfill the demand of grazing animals to prevent grass tetany [4]. Grass tetany is a non-infectious metabolic disorder in cattle and sheep when grazing on cool-season grasses having low Mg concentration or reduced absorption of Mg [5]. This metabolic disorder is caused by a deficiency of available Mg as well as associated available Ca and K contents in forages [6]. Thus, the economic losses due to grass tetany either death or poor growth performance of livestock is a growing concern. Annual losses due to grass tetany in the United States are estimated at $50 to $150 million [7]. Likewise, the grass tetany epidemic in dairy and beef cattle was also responsible for considerable losses in the United Kingdom and other European countries. McCoy et al. [8] reported that grass tetany affected 34% of the animals annually in Ireland. Losses due to grass tetany epidemics were also reported in New Zealand [9] and Australia [10]. In northern Japan, grass tetany incidence is prevalent with cattle grazing on orchardgrass during the autumn and spring seasons [11].
Grass tetany occurs throughout the temperate regions of the world, predominantly while animals are grazing on immature pastures. Several attempts to control or eliminate the incidence of grass tetany have been focused on selective breeding of forages with higher contents of Mg in forage biomass [12]. Among the mineral elements related to grass tetany, Sleper et al. [5] reported that heritability estimates were highest for Mg, intermediate for Ca, and lowest for K contents in most of the C3 forage grass species. Genetic variations of mineral elements and associated mineral ratio, within forage grass species, associated with grass tetany were observed in reed canary grass [13], Italian ryegrass [14] tall fescue [15], wheatgrass [16], orchardgrass [17] [18], and perennial ryegrass [19], respectively.
Tall fescue is one of the major cool-season grasses in American pastureland. Plant breeders developing cultivars to minimize the grass tetany hazards in the United States are concentrating largely on increasing herbage Mg content in tall fescue, and the breeding programs have succeeded in producing “HiMag” tall fescue species [7]. Research efforts led to the breeding of “Magnet” Italian ryegrass [14]. Moseley et al. [20] reported that the use of “Magnet” Italian ryegrass proved effective to minimize the grass tetany incidence (hypomagnesemia) in grazed cattle. Orchardgrass is a commonly grown cultivar in Japan and to minimize grass tetany through forage breeding in Japan, there is a primary requirement and with this view, breeding programs have been conducted to increase Mg content in orchardgrass and “Mgwell” orchardgrass as a high-Mg cultivar was bred by Saiga et al. [21]. Besides, Italian ryegrass, tall fescue, and orchardgrass are popular grasses grown in the Andisol of Japan. However, there is a lack of information on field-based studies associated with the agronomic characteristics and mineral content of high-Mg cultivars compared with commercially available cultivars of Italian ryegrass, tall fescue, and orchardgrass grown in temperate regions of northern Japan. Sabreen et al. [22] reported a higher Mg but lower K uptake in high-Mg cultivars of Italian ryegrass and tall fescue in solution culture, which is further confirmed by this current study under field conditions.
We hypothesized that in Japanese Andisol, all cultivars bred for high-Mg content are expected to balance K optimization with high Mg and Ca uptakes. Our research study was aimed to evaluate the growth, nutrient content, and grass tetany risk of high-Mg cultivars of Italian ryegrass, tall fescue, and orchardgrass grown in Andisols of northern temperate regions of Japan.
2. Materials and Methods
2.1. Experimental Plot, Climate and Soil
A field experiment was conducted at the Iwate University Uadai research farmin Morioka (latitude 39˚42'12" and longitude 141˚09'09"; altitude 141 m from sea level), Japan. While the climate is cold and temperate with uniformly distributed total rainfall (1266 mm) during the year, the winter is freezing cold and partly cloudy (Table 1). Highest precipitation falls from May to September of the year. Mean monthly temperature of more than 23˚C was recorded in August and the lowest of −1.9˚C in January. Relative humidity ranged between 67% and 77% during the growing season.
The field soil is characterized as Umbric Andosol (USDA; Andisol: FAO), with a sandy loam texture, pH 5.8, soil organic matter 18.4 g kg−1, total N 0.39%, available P 6.1 mg∙kg−1, and exchangeable K, Ca, and Mg were 25.8, 410, and 47.0 mg∙kg−1, respectively.
2.2. Test Plant
The test plants were selected as one high-Mg cultivar and three commercial cultivars from Italian ryegrass (Lolium multiflorum L.), tall fescue (Festuca arundinacea Schreb.), and two commercial cultivars from orchardgrass (Dactylis glomerata L.) species, respectively. The “Magnet”, “HiMag” and “Mgwell” were cultivars of Italian ryegrass, tall fescue, and orchardgrass were succeeded by Hides
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Table 1. Climatic data of the experimental area*.
*Japan Meteorological Agency.
and Thomas [14] and bred by Mayland and Sleper [7] and Saiga et al. [21], respectively, as high-Mg containing cultivars. In contrast, commercial cultivars of Italian ryegrass were Ace, Tachiwase, and Waseyutaka, and tall fescue cultivars were Fawn, Hokuryo, and Ky-31, and orchardgrass were Akimidori and Okamidori as those cultivars are commonly cultivated in the northern region of Japan. The morphological and chemical features of all test plants were documented in Table 2.
2.3. Experimental Design and Plant Growth
Prior to establishing the field experiment, one metric ton of dolomite, 140 kg N as urea, 280 kg P2O5, and 140 K2O kg∙ha−1, respectively were applied during the land preparation. The experiment was set up in a split-plot arrangement of randomized complete block design with three grass species as the main plots and the high/low Mg cultivars were split plots with three replications for each treatment combination. Grass seeds were obtained from the Seed Bank of Iwate University, Japan. The seeding rate was 3 gm−2, and the seeds were allowed to lay dormant in the field during the winter months (below freezing point) and the grasses were allowed to grow in the spring throughout autumn. The grasses were reared in the field by following standard pasture management practices.
The plant height of the grasses was recorded each time before the harvest. Grasses were harvested in randomly selected 1 m2 sub-plots in each replicated plot at 6 cm cutting height. Harvesting was performed four times, in May (15 May), July (15 July), September (15 September), and October (15 October), respectively.
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Table 2. Morphological features of cultivars of the forage species*.
*Forage data are generalized from Hides and Thomas [14]; Hides and Lovatt [23]; Kenneth et al. [24]; Mayland and Grunes [6]; Mayland and Sleper [7]; Mayland (personal communication) 2002; Saiga (personal communication) 2000; Saiga et al. [21].
The harvested ground cover was visually assessed for weed species and separated from the grasses. Weeds were then sampled (composite) and their weights were recorded after oven-drying at 60˚C for a 48-h period to calculate weed pressure.
2.4. Nutrient Analysis
After fresh grass samples from each randomly selected microplot (1-m × 1-m) were collected, approximately 500-g samples were taken, oven-dried at 60˚C for 48-h in a forced-air oven. The oven-dried samples were ground to pass thru a 1-mm screen with a cyclone mill followed by making pellets [17] [25], and analyzed for K, Ca, and Mg contents with a live time of 100-sec by energy reflectance X-ray fluorescence spectrometry [26] [27]. The grass tetany index (GTI = [K/(Ca + Mg)] was computed following Kemp and T’Hart [28]. The relative range of each parameter was calculated as:
(1)
2.5. Statistical Analysis
Analysis of variance (ANOVA) was performed for each grass species and harvested with the JMP procedure of the SAS system (SAS Inst., Cary, NC). Treatment means were separated using Tukey’s Honestly Significant Difference (HSD) test when the F-value in the ANOVA was significant at P ≤ 0.05. To determine statistical differences among cultivars, a further ANOVA was performed as a split-plot with the subplots representing the repeated measurements.
While variability within species is anticipated as well as among cultivars of respective forage species, the cultivar/species specific correlation coefficients provide evidence-based knowledge to producers with suitable environmental and site conditions for the specific cultivar to grow. Therefore, we performed species and cultivar specific correlations among the phenotypic traits. To the best of our knowledge, this is the first statement to establish this sort of correlation among morphological and/or physiological traits and nutrient concentrations and their ratio. Pearson phenotypical correlation coefficients were performed (JMP software 4.0, SAS Institute Inc., USA).
3. Results
3.1. Weed Pressure
The dominant weeds were in an order of Rumex obutusifolius ≥ Romexacetosella > Plantago asiatica > Carex lanceolata ≥ Carexnervata, Artemisia princeps > Ixeris dentata > Potentilla freynian > Weigela hortensis, irrespective of grass species or cultivars (Not shown in Table). Among all the cultivars, the highest and lowest infestations of weeds were observed during the July and September growing seasons (Table 3). The weeds such as Rumex obutusifolius and/or Romexacetosella were dominated by Tachiwase, Fawn, and Okamidori species of Italian ryegrass, tall fescue, and orchardgrass, respectively. The average weed pressure among Italian ryegrass cultivars is Tachiwase > Waseyutaka > Ace ≥ Magnet.
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Table 3. Weed percentage of different grass cultivars across four harvests1,2,3).
1)Values for average of 3 transects (1 m × 1 m) from each plot. 2)Values within column and species with the same letter(s) are not significantly different at P ≤ 0.05. 3)Values within row and harvest with the same uppercase letters are not significantly different at P ≤ 0.05. 4)Mean of May to September harvests.
In tall fescue cultivars, the weed pressure was ranked as Fawn > Hokuryo > Ky-31 ≥ HiMag, while in orchardgrass the weed dominance was Okamidori > Akimidori > Mgwell. Across harvests, weed pressure was consistently lower within the high-Mg cultivars than within the commercial cultivars with the relative range of −19.9%, −9.5%, and −13.6% for Italian ryegrass, tall fescue, and orchardgrass species, respectively, suggesting weeds could be controlled by introducing high-Mg grass cultivars in temperate Andisols.
3.2. Plant Development and Biomass Production
The plant height was shorter in high-Mg cultivars compared to commercial cultivars, irrespective of grass species (Table 4). The plant height between high-Mg cultivars and commercial cultivars ranked as orchardgrass > Italian ryegrass > tall fescue. The plant height was tallest in the July harvest and the lowest in the October harvest, regardless of grass species or cultivars.
While the dry-matter yield of grass cultivars varied among the harvests (Table 5) and recorded the lowest seasonal yield in high-Mg containing cultivars when compared to commercial cultivars with the relative range −10%, −5.2%, and −8.7%, for Italian ryegrass, tall fescue, and orchardgrass respectively. Results indicated that when compared with both Magnet Italian ryegrass and Mgwell orchardgrass, the HiMag tall fescue responded better in dry-matter production,
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Table 4. Plant height (cm) of different grass species across four harvests1,2,3).
1)Values for average of 10 plants from each plot. 2)Values within column and species with the same lowercase letters are not significantly different at P ≤ 0.05. 3)Values within row and harvest with the same uppercase letters are not significantly different at P ≤ 0.05. 4)Mean of May to September harvests.
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Table 5. Dry matter yield (tha−1) of different grass species across four harvests1,2,3).
1)Values within column and species with the same lowercase letters are not significantly different at P ≤ 0.05. 2)Values within row and harvest with the same uppercase letters are not significantly different at P ≤ 0.05. 3)Values for Italian ryegrass includes reseeding plants from the original plants. 4)Total of May to September harvests.
while the seasonal yields ranked as Magnet > HiMag > Mgwell. Among the grasses, the total forage yield was highest in Italian ryegrass followed by tall fescue, and the lowest in orchardgrass, irrespective of cultivars. Likewise, the highest dry-matter yield was recorded in the May harvest for Italian ryegrass and in the October harvest for orchardgrass. For cultivars of tall fescue, the yield decreased from May to the July harvests and then continued to increase in the September harvest.
3.3. Potassium, Calcium and Magnesium Concentration
The highest K content for Italian ryegrass was observed in September which can be attributed to the low dry-matter yield for that harvest (Table 6). Regardless of the cultivars of tall fescue and orchardgrass species, the highest K content was recorded in October harvests which can be partially associated with the low dry-matter yield for that harvest. Among the Italian ryegrass species, the highest K content was determined in Waseyutaka and the lowest was in Tachiwase over four harvests. Likewise, the highest K content was observed in Ky-31 and the lowest was in Hokuryo cultivars in tall fescue species across the harvests. For orchardgrass species, the cultivar Okamidori showed the highest and Mgwell showed the lowest K content across harvests. Tall fescue cultivar Hokuryo showed a lower K content than the other three cultivars across the harvests. Irrespective of grass
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Table 6. Potassium content (% dry matter) of different grass species across four harvests1,2,3).
1)Values within column and species with the same lowercase letters are not significantly different at P ≤ 0.05. 2)Values within row and harvest with the same uppercase letters are not significantly different at P ≤ 0.05. 3)Values for Italian ryegrass includes reseeding plants from the original plants. 4)Mean of May to September harvests.
species and cultivars, the mean K content was lowest in Okamidori (2.72% DM) while the highest was in Ky-31 (4.42% DM). Among all cultivars, Mgwell cultivars accumulated less K than that of other high-Mg cultivars regardless of the species. Across the harvests, tall fescue HiMag and orchardgrass Mgwell showed lower K content when compared to their commercial cultivars with a relative range of −1.99% and −2.24%, respectively. The Waseyutaka showed the highest K content among all cultivars of Italian ryegrass; however, over the four harvests, Magnet showed higher K content than that of the commercial cultivars with a relative range of 0.34%.
In Italian ryegrass Waseyutaka, and all cultivars of tall fescue, Ca content was increased in the July harvest compared to the May harvest and then decreased in the September and October harvests, respectively (Table 7). In other words, the lowest Ca content in tall fescue was recorded in the September harvest when compared to the highest Ca content was recorded in the October harvest. The Ca content increased gradually over time in all cultivars of orchardgrass species and in the Ace cultivar of tall fescue. The Magnet Italian ryegrass, HiMag tall fescue, and Mgwell orchardgrass cultivars showed a significant increase in Ca content than others. Mean forage Ca content varied from 0.35% to 0.53% across harvests, irrespective of grass cultivars. The Ca content averaged across harvests for tall
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Table 7. Calcium content (% dry matter) of different grass species across four harvests1,2,3).
1)Values within column and species with the same lowercase letters are not significantly different at P ≤ 0.05. 2)Values within row and harvest with the same uppercase letters are not significantly different at P ≤ 0.05. 3)Values for Italian ryegrass includes reseeding plants from the original plants. 4)Mean of May to September harvests.
fescue was 0.35% to 0.44% and for orchardgrass 0.41% to 0.48%, respectively, while Italian ryegrass ranged between 0.51% to 0.68%.
Across all harvests, the HighMg cultivars showed a significantly higher Mg content when compared with the commercial cultivars of respective species (Table 8). Cultivars of Italian ryegrass had highest Mg content in the September harvest while having the lowest in the May harvest. For tall fescue, the highest Mg values were observed in the October harvest while the lowest values were recorded in the May harvest. However, there was no significant difference in content Mg among the tall fescue cultivars between the July and September harvests. Among the orchardgrass cultivars, the lowest and highest Mg content in forages were determined in the May and October harvests, respectively.
3.4. Grass Tetany Index [K/(Ca + Mg)]
The K/(Ca + Mg) was calculated on an equivalent molecular weight basis to evaluate the balance of K content against Ca and Mg contents and these values indicate the potential risk of grass tetany as caused by forage grasses (Table 9). The values for the equivalent ratio of tetany risk were highest in the September harvest and lowest in the July harvest for all cultivars of Italian ryegrass species. The lowest grass tetany ratio was recorded in October for tall fescue and July for orchardgrass, irrespective of cultivars. While considering different species for
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Table 8. Magnesium content (% dry matter) of different grass species across four harvests1,2,3).
1)Values within column and species with the same lowercase letters are not significantly different at P ≤ 0.05. 2)Values within row and harvest with the same uppercase letters are not significantly different at P ≤ 0.05. 3)Values for Italian ryegrass includes reseeding plants from the original plants. 4)Mean of May to September harvests.
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Table 9. Equivalent ratio of K/(Ca + Mg) of different grass species across four harvests1,2,3).
1)Calculated on an equivalent per kilogram basis. 2)Values within column and species with the same lowercase letters are not significantly different at P ≤ 0.05. 3)Values within row and harvest with the same uppercase letters are not significantly different at P ≤ 0.05. 4)Mean of May to September harvests.
grazing it seems that orchardgrass has less tetany risk throughout the season. For high Mg containing cultivars of Italian ryegrass (Magnet) and tall fescue (HiMag), the equivalent ratios fluctuated with harvests; however, the values were lower than the threshold values of grass tetany risk compared to the commercial cultivars. In the autumn harvest (September), the K/(Ca + Mg) values posed a risk of grass tetany in the HiMag cultivar of Italian ryegrass and tall fescue, respectively.
The Ace Italian ryegrass showed lower tetany values than the critical level of equivalent ratio (2.2) only in the October harvest. In contrast, the Tachiwase and Waseyutaka (commercial cultivars) which are commonly used as forages in Japan showed lower tetany values than the critical level of equivalent ratio only in May and July harvests. The Fawn, Hokuryo and Ky-31 cultivars of tall fescue species showed grass tetany risk in May and September harvests. The values of relative range for grass tetany risk were recorded in orchardgrass (−22.9%) followed by tall fescue (−21.3%) and the lowest in Italian ryegrass (−12.9%), respectively.
3.5. Phenotypic Correlation
The plant height of tall fescue positively and significantly correlated with the grass tetany index (GTI). Likewise, the HighMg cultivar dry matter positively correlated with the GTI. Moreover, a species-specific correlation was observed between K content and the GTI. In Italian ryegrass species, K content positively correlated with Mg(r = 0.735) and the GTI (r = 0.682). However, in tall fescue, K content positively correlated with Mg content (r = 0.630) but negatively correlated with the GTI (r = −0.337). In commercial cultivars, K content positively correlated with Mg content but poorly correlated with the GTI (r = 0.284). While in high Mg cultivars, K content significantly correlated with the GTI (r = 0.620), it did not correlate significantly with Mg content (r = 0.279). In orchardgrass, K content moderately correlated with Ca (r = 0.654) and the Mg (r = 0.409). However, the correlation between K content was very weak and insignificant with GTI (r = 0.052) in orchardgrass. In commercial cultivars, K content was significantly correlated with Mg content (r = 0.629) but not with the GTI (r = 0.284). In contrast, K content significantly and positively correlated with the GTI (r = 0.620) but not with Mg content (r = 0.279) in the high Mg cultivars. Likewise, Ca content significantly correlated with the GTI among all species and cultivars except the High Mg cultivar. Regardless of the grass species and cultivars, a positive correlation was observed between Ca content and Mg content. Grass tetany index negatively correlated with Mg and Ca regardless of species (Table 10).
4. Discussion
4.1. Weed Pressure
A significantly reduced weeds pressure has shown how undesirable and highly invasive weeds could be controlled by introducing highMg grass cultivars in
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Table 10. Correlation among phenotypic traits of cool season grasses from pooled data over four harvests*.
*Shaded indicates statistically significance at P ≤ 0.05.
temperate Andisols. Weeds especially docks (Rumex sp.) are one of the highly adaptive ones predominant in intensive grassland management systems globally [29]. Haggar [30] reported that docks were most prevalent and associated with the frequent usage of chemical fertilizers. As our grass management systems were based on chemical fertilizers, i.e., increasing infestations of docks were judged [31].
4.2. Plant Height and Biomass Production
A significantly lower amount of dry matter produced by High Mg cultivars when compared to commercial cultivars was reported in other countries as the herbage yield depends on nutrient availability to support adequate nutrition and growth [32] [33]. Irrespective of cultivars, the annual dry-matter production of Italian ryegrass (14.7-ton∙ha−1), tall fescue (13.3-ton∙ha−1), and orchardgrass (5.3-ton∙ha−1) seemed lower in Andisol than other soils regardless of species or cultivars when compared with dry-matter production of tall fescue [34], Italian ryegrass, and orchardgrass [35], respectively. Rahman and Saiga [17] reported 6.4-to-14.1-ton∙ha−1∙y−1 biomass production of orchardgrass grown in Andisol under different management practices. The total annual dry matter production of forage mixtures was about 13 to 16-ton∙ha−1 in red brown soil under slurry application [36]. In Andisol of New Zealand, up to 20-tondry dry-matter, ha−1 of pasture yield was recorded under suitable climatic and site conditions [37]. Both biotic- and abiotic stresses, as well as the microclimate, may be responsible for the lower production of aboveground biomass of cool-season grasses in the temperate region of Japan. A supplement of Mg significantly increased the yield in grass species, concurrently declining concentrations of crude protein, Ca, sodium (Na), manganese (Mn), and K/Mg and Ca/Mg as the dilution effect of biomass increased with increased Mg content [38].
4.3. Crown Nutrients
While the Mg contents among the cultivars of different species were similar, the HiMg tall fescue and Mgwell orchardgrass had lower K content than with their respective commercial cultivars. It is reported that the absorption of Ca and Mg by ruminants decreased with increased K levels in forage [33]. Smith et al. [39] suggested that when forage K content is ≥25 g∙kg−1 dry-matter, the Mg content decreased to 1.9 g∙kg−1. Similarly, when the forage K content increased to 65 g∙kg−1, the Ca content decreased to 6 g∙kg−1. Sleper et al. [5] indicated that the correlations between herbage yield and K, Ca, and Mg contents were low in tall fescue. In support of our results, Wilkinson and Mayland [40] reported that the total yield of HiMag tall fescue was 11% less than that of Ky-31. When the high Mg Italian ryegrass cultivar Magnet was compared with other cultivars, its dry-matter yield was between 8% and 10% lower than the higher yielding cultivar [41]. Then one significant difference in yield between high Mg and commercial cultivars indicated that the cultivars bred for high Mg contents did not compensate their yield under temperate climates. While mean K content varied between 2.72 (Okamidori) to 4.42% (Ky-31), which is accepted as adequate for sheep nutrition [42], but inadequate for cattle [43] and deer [44] nutrient requirements. The values are low to support adequate vegetative growth of grasses [33] [32]; however, are nontoxic for herbage quality [45]. Among all cultivars, HighMg cultivars showed a low relative range suggesting that HighMg cultivars accumulated lower K than other commercial cultivars which was the main focus of the breeding program.
The mean forage Ca content varied between 0.35% to 0.53% across harvests, irrespective of grass cultivars which are considered inadequate for sheep [42], cattle [32], and deer [46] nutrition. However, the Ca content averaged across harvests was adequate in tall fescue and orchardgrass, respectively, while in Italian ryegrass its content was higher for grasses [47]. The Mg content increased consistently across the season which is below the normal levels of forage grasses [39]. These high Mg-containing cultivars had higher Ca content which had consequently lower K/(Ca + Mg) with minimum risks of grass tetany [48]. A significant variation in Ca content in cool-season grasses across harvests observed in our studies collaborates with the findings of Jones and Tracy [49]. They reported a similar phenomenon of variation in herbage mineral contents throughout the growing season. A significantly higher Ca and Mg content across harvests recorded in high Mg cultivars compared to commercial cultivars, irrespective of species justifies the breeding of high Mg cool-season grass cultivars. The grasses bred for high Mg content are anticipated to balance K optimization with high Mg and Ca uptakes which could minimize grass tetany risks in temperate regions of the world including Japan. The lowest nutrient content was recorded in the May harvest, regardless of species and cultivars may be associated with the rapid vegetative growth of grasses in the spring. The Energy Dispersive X-ray Microanalysis showed that Mg density in xylem had higher Mg content across two harvests and higher Ca and lower K contents for HiMag tall fescue when compared to commercial tall fescue cultivar Ky-31 [50]. The HiMag was able to translocation more Mg from root to shoot than the commercial cultivars. A doubling of the K in nutrient solution decreased Mg content and increased K/Mg in roots but did not significantly affect Mg contents in crowns nor leaves. Increasing the Mg in solution increased Mg content in roots, crowns, and leaves. This may be evidenced associated with several metabolic processes that limit K uptake and possibly favor a greater Mg translocation in all tall fescue cultivars [34]. Sabreen et al. [51] reported a similar trend in separate studies with optimized nutrient solutions using increased K concentration in nutrient solutions [52], where three cool-season High Mg cultivars of Italian ryegrass, tall fescue, and orchardgrass were compared with commercial cultivars of respective species.
4.4. Grass Tetany Index [K/(Ca + Mg)]
The grass tetany index, [K/(Ca + Mg)] has shown fluctuations with harvests, and the index of 1.4 was recorded in Mgwell (orchardgrass), 1.90 in HiMag (tall fescue), and 2.00 in Magnet (Italian ryegrass), respectively across harvests. Among the species, the lowest K/(Ca + Mg) was observed for Mgwell which is bred by Saiga et al. [21]. It appears that the values of grass tetany ratio [K/(Ca + Mg)]were lower than the threshold values in all high Mg cultivars, suggesting that all cultivars breed for high Mg content are safe for grazing ruminants throughout the seasons in temperate region globally including Japan. Moseley and Baker [41] evaluated the Magnet Italian ryegrass cultivar (Bb2067) for its efficacy in alleviating the incidence of hypomagnesemia in lactating ewes by comparing it with the control pasture cv. RvP, and Magnet (Italian ryegrass) maintained higher Mg contents throughout the season. High levels of hypomagnesemia and fatalities were reported in animals grazing RvP control cultivar than those on Magnet (Italian ryegrass) pasture, which was corroborated by differences in serum Mg concentration. Moseley and Baker [41] concluded that the use of a high Mg grass cultivar consistently proved its effectiveness to reduce hypomagnesemia under intensive grazing. A high Mg Italian ryegrass had a consistently greater Mg content associated with lower K content and grass tetany index than the low Mg cultivar (Bb1276). They reported that the high Mg content was not related to forage yields in Aberystwyth (Wales) and Edinburgh (Scotland). Therefore, the Bb2067 cultivar was assumed a promising cultivar for reducing grass tetany, and the cultivar was enclosed for breeding programs [53]. Binnie et al. [19] compared perennial ryegrass (Ramore) bred for high Mg content with a control cultivar, aiming for its ability to reduce the grass tetany incidence. They suggested that under grazing high Mg cultivars provide a proactive management strategy to reduce the grass tetany incidence. All these observations are in collaboration with our results.
Rahman and Saiga [17] studied the impact of dairy manure on mineral nutrient uptake patterns with a special reference to tetany potential risk by orchardgrass grown in Andisol of Japan. They observed that the uptake of grass tetany-related mineral nutrients was greater in dairy manure amendments than that of the chemical fertilizers and the uptake of mineral nutrients in forages was directly proportionate to grass tetany risk. This demonstrated that soil amendments (viz. slurry, compost, vermicompost, biochar) might change grass tetany potentiality as they alter soil oxygen diffusion resulting in impeding gas exchange processes by water-saturation of soil pores. Introducing grass with biological modifications (endophyte infection) showed inconsistent results on the uptake of mineral nutrients with soil types and plant ecotypes [54] [18]. Therefore, breeding grass could be the other possibility to minimize grass tetany risk.
The HiMag tall fescue is a cultivar selected for increased Mg and Ca uptake to reduce the K/(Ca + Mg). Wilkinson and Mayland [40] compared the HiMag with other tall fescue cultivars for mineral concentration and their results showed that the shoot Mg and Ca contents were greater and the K/(Ca + Mg) was lower for HiMag than the Ky-31. Crawford et al. [55] reported that cattle grazing of HiMag is likely had a reduced risk of grass tetany. Our results showed the same phenomenon for Magnet Italian ryegrass, HiMag tall fescue, and orchardgrass grown in temperate Andisol. However, the K/(Ca + Mg) for Magnet Italian ryegrass and the HiMag tall fescue were 2.12 in the October harvest and 2.16 in the September harvest, respectively, which is very close to grass tetany risk. It can be inferred that Magnet Italian ryegrass and HiMag fescue could be a grass tetany risk for ruminants during October and September harvests, respectively, while growing in temperate Andisols globally including Japan. The higher shoot Mg content and the lower K/(Ca + Mg) in high Mg-containing cultivars proved that there is significant potential to breed forages with a balanced uptake of nutrients (K, Ca, and Mg) to improve animal health hazards. The relative range values for grass tetany risk between high Mg and commercial cultivars were negative for all species which indicates that the selection of high Mg grasses via breeding is justifiable [56]. Our results suggested that breeding grass species for reducing grass tetany are reliable for temperate regions of Japanese Andisol.
4.5. Relationship among the Plant Traits
As one of the macronutrients essential for plant growth, K plays important physiological roles in photosynthesis and stomatal control of transpiration and thus, participates in other biochemical roles in protein synthesis [1]. Therefore, a significant correlation of K and Mg contents with forage dry matter production would be expected; however, we did not observe any correlations of K with dry-matter production in our experiments. While the Mg content was negatively correlated with dry-matter production in commercial cultivars and high Mg cultivar, the Mg and Ca contents negatively and K positively correlated with the GTI as expected. Similar relations were reported by other studies [16] [57] [58]. In high Mg cultivars, the K content positively and Mg content negatively correlated with the GTI, which suggests that the selection of the High Mg cultivar had a positive effect on grass tetany risk in ruminants.
5. Conclusion
Field research in the USA, Europe, Australia, and New Zealand revealed that high Mg containing Italian ryegrass, tall fescue, and orchardgrass bred by scientists had significantly higher shoot Mg concentrations than commercial cultivars. These high Mg-containing cultivars were also higher in Ca concentration, and consequently reduced K concentration, resulting in a lower K(Ca + Mg) ratio than the commercial cultivars. We evaluated those species of high magnesium-containing cultivars with commercials cultivars for grass tetany risk in the Japanese climate. Results showed that the high Mg Italian ryegrass, tall fescue, and orchardgrass breeds had consistently higher shoot Mg content than that of the commercial cultivars. These high Mg cultivars had a higher Ca content, which was consequently associated with K content with a resulting decrease in grass tetany risk than the commercial cultivars grown in temperate Andisols. It could be conferred that the tetany index in grass species is very much age (harvest) specific and microclimate and site-dependent. While there is a possibility that high Mg cultivars moderately perform to dry-matter productivity, further research on dry-matter productivity of high Mg forage is required.
Acknowledgements
The authors would like to thank Dr. H.F. Mayland (USDA-ARS, Northwest Irrigation and Soils Research Lab., Kimberly, USA) for his cooperation and support during the experiment. We also thank Bradford Sherman at The Ohio State University to review and edit the manuscript.