INTRODUCTION

Italian ryegrass (Lolium multiflorum Lam.; IRG) is a winter forage crop that, in Korea, is sown in late September and harvested in May. It is widely cultivated because of its versatility as a forage crop; it can be used as fresh-cut fodder, silage, or hay. Historically, IRG has been cultivated mainly in the southern regions of Korea as it is a suitable paddy field crop because of its excellent livestock feed value and palatability, and its capacity to grow under wet conditions (Lee et al., 2020). Although there are many advantages to growing IRG in paddy fields, in order to expand the cultivation area to the central and northern regions of Korea, it is necessary to overcome the problem of winter survival of this forage crop (Lee and Kim, 2017). Recently, cold tolerant and early maturing IRG varieties such as ‘Kowinearly’, ‘Greenfarm’, and ‘Kospeed’ have been developed, and they are cultivated in the central region (Choi et al., 2007;Choi et al., 2011;Ji et al., 2011;Kim et al., 2015). With the development of new varieties suitable for the domestic environment, the cultivated area of IRG in Korea has more than tripled in the last 10 years (2009-2019) from 52,000 to 165,000 ha, and accounts for 87% of the total winter forage crop production (MAFRA, 2020).

In Korea, the IRG sowing time recommended by the standard cultivation technique is from late September to early October in the central region and early to mid-October in the southern region (Choi et al., 2017). However, in the central and northern regions of Korea, where IRG is sown in paddy fields after the rice harvest, sowing is not possible in mid-to-late September (the appropriate sowing season), and winter survival of IRG is poor, resulting in a decrease in yield (Kim et al., 2015). Recent developments in IRG sowing technology have enabled cultivation of this forage crop under growing rice; consequently, in the central region, IRG can be sown and cultivated at the appropriate time in late September before the rice crop is harvested (Kim, 2016a). However, if the rice harvest is delayed after the IRG has been sown, the forage plants experience stunted growth and enter winter with a low tiller number and short plant length, resulting in a decrease in the winter survival rate and thus the yield of the IRG (Kim et al., 2016b).

In order to improve the stability of IRG forage production and the yield per unit area in central Korea, it is important to select a suitable cultivation site and IRG variety, and to apply appropriate cultivation technology. Among possible IRG varieties, it is most important to select varieties that are cold tolerant (Park et al., 1987). Consequently, for successful cultivation of IRG in central Korea, a domestically developed variety with a high winter survival rate and early maturity should be selected (Choi et al., 2018).

The growth characteristics and yield of ‘Kowinearly’, an early maturing IRG variety, in paddy fields have been studied to determine the optimal time to harvest rice undersown with this IGR variety (Kim et al., 2016b). However, no studies have been conducted on the extremely early maturing variety ‘Greenfarm’ currently cultivated on many farms in central Korea. Therefore, the aim of this study was to obtain basic cultivation management data for ‘Greenfarm’ by investigating the growth characteristics and dry matter yield of this IRG variety before and after winter, particularly in response to the timing of rice harvesting after sowing of this extremely early maturing IRG variety under growing rice.

Materials and Methods

Plant Materials, Cultivation, and Growth Conditions

The study was conducted at the National Institute of Animal Science (NIAS) in Cheonan, central Korea (N 37°55’58”, E 127°05’36”). An experimental paddy field was cultivated for three consecutive growing seasons: 2011–2012, 2012–2013, and 2013–2014 to evaluate the growth characteristics and forage productivity of IRG in response to the time of rice harvest after sowing of the IRG under the growing rice The IRG was sown on September 27 (first year), September 28 (second year), and September 27 (third year) under growing rice before the rice harvest. The amount of seed sown was 50 kg/ha, and the field size of each treatment was 1,000 m². After the IRG was sown in late September, the rice crop was harvested at intervals of 10 days (treatment 1), 20 days (treatment 2), and 30 days (treatment 3) after the date of IRG sowing. The rice straw was immediately collected and fertilized, and a drainage channel was installed to block wet damage. Basal fertilization using N:P₂O₅:K₂O at 40:75:75 kg/ha was conducted and additional fertilizer was applied at the end of February (N:P₂O₅:K₂O at 100:75:75 kg/ha). Other cultivation management strategies were in accordance with standard IRG cultivation methods based from recommendations of Rural Development Administration (RDA).

Average monthly temperature and precipitation data for the experimental site are shown in Table 1. The temperature in December 2012 was −3.5°C and −4.0°C lower compared to that in December 2011 and 2013, respectively, and in January 2013 it was also lower by −0.9°C and −2.8°C compared to that in January 2012 and 2014, respectively. From December 2012 to April 2013, a low temperature phenomenon occurred; however, the winter of 2013 and the spring of 2014 were relatively warm. During the winter of 2011, the early winter was warm, causing over-growth, and there was a severe drought until the spring of 2012. The drought was not severe from the winter of 2012 to the spring of 2013, but the amount of precipitation was insufficient. During the spring of 2014, there was also insufficient precipitation. The average temperature in the autumn (October to November) after sowing from 2011 to 2013 was 11.4°C, 9.6°C, and 10.5°C, respectively, which was similar in 2012 (9.8°C), and higher by 1°C in 2011 and 2013 (National Weather Service, Korea).

Investigation of Italian Ryegrass Growth Characteristics and Yield

The IRG tiller number, plant length, winter survival rate, and heading date were investigated for each treatment during each growing season. The number of tillers before winter was determined in mid-December, the winter survival rate in mid-March, and plant length in mid-December and at the time of IRG harvest. The tiller number and plant length were also assessed in early December and early March, and the winter survival rate in early March. The fresh yield was calculated at the harvest per 1 m² at three locations in each treatment field. In order to calculate dry matter yield content, the samples collected at each harvest were dried at 65°C for 72 hours. The dry matter yield was calculated by multiplying the ratio of dry matter with fresh yield, and the value was converted to dry matter per hectare. For statistical analysis of the data, analysis of variance was performed using the SAS software package (2002), and all data were subjected to analysis using Duncan’s multiple range test. Significance was set at the 5% level.

Results and Discussion

Growth Characteristics Before Winter and Winter Survival of Italian Ryegrass

The tiller number, plant length, and winter survival rate after sowing of the IRG ‘Greenfarm’ variety are presented in Table 2. A comparison of the pre-winter growth characteristics indicated that a delay in rice harvesting negatively impacted the number of tillers and plant length as the tiller number before winter decreased significantly from 5.0 (treatment 1) to 3.4 (treatment 2) to 2.4 (treatment 3), and the plant length decreased significantly from 19.6 cm (treatment 1) to 18.1 cm (treatment 2) to 11.1 cm (treatment 3) (p < 0.05; Table 2). In particular, the plant length decreased by 35.5% and 68.7% in 2012–2013, which was a higher reduction compared to that of 19% and 40% for the early maturing variety ‘Kowinearly’ (Kim et al., 2016b). This may be because the average pre-winter temperature in December 2012 was the lowest average temperature recorded in 30 years. The average IRG plant length when the rice was harvested after 20 days was only 7.7% shorter compared to the plant length when rice was harvested after 10 days, but was 43.4% shorter when the rice was harvested after 30 days. Kim et al. (2016b) have reported that the early maturing variety ‘Kowinearly’ showed reductions in plant length of 12.7% and 33.9%, when the rice was harvested after 20 and 30 days, respectively, compared to when the rice was harvested 10 days after IRG sowing. These results suggest that when IRG is sown under growing rice, the pre-winter IRG growth is slower when the rice harvest is delayed, and the ‘Greenfarm’ variety, which has relatively weak cold-tolerance, has less growth before winter compared to the cold tolerant variety ‘Kowinearly’ because the rice is harvested later after sowing (Choi et al., 2018).

The average winter survival rate was highest for treatment 1 (84%) but decreased significantly to 77% for treatments 2 and 3 with rice harvesting delays (p < 0.05). In particular, the IRG plants in treatment 3 had a relatively high winter survival rate during the 2013–2014 growing season (Table 2). The number of tillers and the plant length before winter are factors that increase the winter survival rate, and they are closely related to the temperature and precipitation during the autumn growth period after sowing (Kim et al., 2009). The precipitation in October 2011, 2012, and 2013 was 26.5, 66.6, and 13.8 mm, respectively. In October 2013, the IRG plants in treatments 1 and 2, with earlier rice harvesting, suffered drought damage, but in treatment 3, the winter survival rate was relatively high, possibly a result of the effect of prevention of transpiration of water by the standing rice. The general technique of sowing IGR under stands of growing rice is to sow when the soil moisture is sufficient, immediately after the final dripping while the rice is growing in the paddy field (Kim et al., 2016c). Considering the growth characteristics of IRG, particularly excellent soil moisture tolerance and good growth in areas with relatively high temperatures (Kim et al., 1991), the findings for sowing IRG into the stands of growing rice in the autumn of 2013 and harvesting the rice 30 days after sowing the IRG indicate appropriate growing conditions.

Dry Matter Yield of Italian Ryegrass at Different Rice Harvesting Times

The heading date, plant length, and dry matter yield of the IRG ‘Greenfarm’ variety in response to rice harvesting times are shown in Table 3. In 2012, a delay in rice harvesting reduced the IRG dry matter yield by 3% and 8% in treatment 2 and treatment 3, respectively. A similar finding was obtained in 2013 where, compared to treatment 1, the dry matter yield of treatments 2 and 3 decreased by 8% and 20%, respectively. However, in 2014, the delay in rice harvesting resulted in an IRG dry matter yield decrease of 8% for treatment 2 but a 6% increase for treatment 3. Regarding the three-year average from 2012 to 2014, compared to treatment 1, treatments 2 and 3 reduced the IRG dry matter yield by 6% and 7%, respectively (p < 0.05; Table 3). The dry matter yield of the IRG ‘Greenfarm’ variety was affected more by the winter survival rate than by the number of tillers and plant length before winter (Table 1 and 3). The IRG heading dates were May 7^th in 2012, May 13^th in 2013, and May 1^st in 2014, indicating that there was no difference in heading dates in response to rice harvesting time within each growing season. The difference in heading dates across the three years of the experiment was most likely because heading date varies depending on the growth level during the winter period and the temperature, precipitation, and amount of fertilization during the spring growth period (Kim et al., 2016).

적 요

본 연구는 이탈리안 라이그라스를 벼 입모중 파종 후 벼 수 확시기에 따른 이탈리안 라이그라스 극조생종 품종 ‘그린팜’ 의 생육특성 및 생산성을 평가하기 위해 중부지역에 위치한 천안에서 2011–2012, 2012–2013 및 2013–2014까지 3년간 수행하였다.