| Annual forages play a critical role in agricultural production and improvement of ecosystems and the environment.Sowing date is one of the most important methods in regulating agricultural management and it has a strong impact on optimising forage yield and the efficiency of resource usage.A three-year field experiment was conducted at the Quzi experimental station of Lanzhou University in Huan county,Gansu province of the Longdong Loess Plateau during 2011-2013.Maize?Zea mays?,Sudan grass?Sorghum sudanense?,foxtail millet?Setaria italic?and Japanese millet?Echinochloa crusgalli?L.?Beauv.var.frumentacen?were selected as the forage species.The response of the four forage species to early?mid May?,middle?late May?and late?mid June?sowing was investigated in regards to productivity,water and nutrient use efficiency,as well as crop quality.The aim of the study was to provide reference information for the development of grassland agriculture.The main results were as follows:1.Sowing date had a big influence on the hay yield of the four forage species and for biomass partition.At the flowering stage,the highest yield was with Sudan grass(6.5 t ha-1),followed by maize(4.5 t ha-1),Japanese millet(4.4 t ha-1)and foxtail millet(3.9 t ha-1).Maize,foxtail millet and Japanese millet produced the highest yield in the early sowing,which was 9%,30%and 38%higher than for the late sowing.Sudan grass had the highest yield in the medium sowing treatment,which was 32%and 22%higher when compared to early and late sowing treatments.Component biomass of forage leaf,sheath,stem and spike were measured at the full-flowering stage.The results showed that stem biomass contributed the most for maize(176 g m-2)and Sudan grass(290 g m-2).The leaf biomass was highest for foxtail millet(140 g m-2),while leaf(117 g m-2)and stem biomasses(128 g m-2)were the main contributors for Japanese millet.In terms of sowing date,spike biomass of maize of the early sowing was 46%higher than the late sowing.The leaf,sheath and stem biomasses at foxtail millet and Japanese millet of the early sowing were 30%,19%,22%and 51%,40%,55%higher when compared to late sowing,respectively.With medium sowing,stem and spike biomass of Sudan grass was increased by29%,33%and 44%,58%compared with early and late sowing treatments,respectively.2.Sowing date plays an important role in the thermal time and canopy development of the four forage species.The thermal times for growth of foxtail millet,Japanese millet,maize and Sudan grass were 1357?d,1343?d,1334?d and1083?d,respectively.Emergence of maize,Sudan grass,foxtail millet and Japanese millet needed 18.9?d,29.1?d,23.7?d and 21.3?d more thermal time in the early sowing compared to late sowing.In contrast,the time from emergence to flowering increased 101.9?d,33.0?d,71.9?d and 181.3?d,respectively.Leaf number on the main culm of maize,foxtail millet and Japanese millet in the early sowing treatment was 1.9,7.8 and 3.5 more than for the late sowing caused by thermal time.The phyllocron of Japanese millet and Sudan grass were 86.8?d and 80.0?d,higher than foxtail millet?66.3?d?and maize?54.7?d?.The phyllocron of the forage species was determined by both thermal time and day length.Radiation use efficiency?RUE?of Sudan grass,foxtail millet and Japanese millet were lower in the early sowing,with a decrease of 83%,55%and 29%compared with the late sowing treatment,respectively.3.Sowing date changed water use efficiency and strategy of the forage species.Among the four species,the highest water use efficiency was observed with maize(43.4 DM ha-1 mm-1),followed by Sudan grass(26.5 DM ha-1 mm-1),Japanese millet(26.2 DM ha-1 mm-1)and foxtail millet(24.6 DM ha-1 mm-1),respectively.Early sowing increased water use efficiency of maize and foxtail millet by 33%and 42%,respectively.Improved water use efficiency for Sudan grass was observed in the medium sowing,which was 20%and 38%higher than the early and late sowings,respectively.Water use strategy of the four forage species at different growing stages were analysed by stable isotope method.The four forage species tended to absorb water from a deep layer and the depth of water uptake increased from jointing,flowering to maturity.At the jointing and flowering stages,maize,foxtail millet and Japanese millet mainly used soil water at 0-0.6 m,however,as the forage species developed to maturity,maize and foxtail millet took water from 0.1-1.2 m,while Japanese millet still used water at 0.1-0.6 m.In contrast,Sudan grass at jointing and flowering stages mainly absorbed water at 0.1-1.2 m,while it used water at 0-0.6 m at maturity.With the delay of sowing date,the ratio of water from 0.1-0.6 m and 0.6-1.2 m layers used by the four forage species at flowering and maturity stages was increased.4.Sowing date changed nitrogen accumulation aboveground and the forage quality of the four forage species.The highest nitrogen accumulation was found with maize(90.1 kg ha-1),the other three forages were in the order,Japanese millet(57.3kg ha-1),Sudan grass(52.2 kg ha-1)and foxtail millet(50.7 ka ha-1).Early sowing improved nitrogen accumulation for maize,foxtail millet and Japanese millet by 10%,38%and 54%compared with late sowing,respectively.Medium sowing was 33%and 35%higher compared to the early and late sowing treatments in nitrogen accumulation for Sudan grass.Nitrogen accumulation of the stem of maize was the highest under the early sowing,which was 25%more than for the late sowing;the corresponding value of the spike of foxtail millet increased by 57%,while that of leaf,sheath,stem and spike of Japanese millet was improved by 39%,26%,51%and63%,respectively.Crude protein content at the full-flowering stage was the highest with Sudan grass?8.7%?,followed by maize?7.7%?,Japanese millet?7.7%?and foxtail millet?7.2%?.The effect of sowing date on the aboveground biomass of maize and foxtail millet were minor.Early sowing increased the crude protein content of Japanese millet by 29.7%.Late sowing improved the crude protein content of Sudan grass by 11.6%.The highest value of soil mineral nitrogen content was with maize(40.0 kg ha-1),followed by Sudan grass(31.5 kg ha-1),foxtail millet(32.0 kg ha-1)and Japanese millet(28.5 kg ha-1).Soil mineral nitrogen increased with the delay of sowing.Early sowing decreased soil mineral nitrogen by 29%,24%,38%and 14%,respectively.Sowing date could influence soil mineral nitrogen content.For maize and Sudan grass this occurred at 0-1.2 m,for foxtail millet at 0.6-1.2 m,and for Japanese millet at0-0.6 m.This study that investigated the growth and resource use of maize,Sudan grass,foxtail millet and Japanese millet provided valuable information.However,future work should further explore the mechanism of the increase in yield,and combine sowing date with other management methods to further improve the productivity of annual forage species. |
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