3.1. Effects of Planting Date and Film Mulching on Heat Distribution
3.1.1. Soil Temperature
The dynamic changes of daily soil temperature (DST) in the depth of 5–25 cm under film mulching and in bare land during maize growing season in 2015 and 2016 are shown in Figure 4
. The increasing rate of the average DST under film mulching at various GPs, compared with those of bare land in different treatments, are shown in Figure 5
From MD1 to MD4, the accumulated soil temperature (AST) under mulching during maize entire GP for two years averagely increased by 149.9 °C, 136.6 °C, 116.5 °C, and 99.6 °C, respectively, compared with those in bare land. The AST increments of seedling stage in MD1–MD4 accounted for 59.7%, 59.1%, 55.0%, and 54.8% of those during the total GP, respectively. Results indicated that the warming effect of film mulching decreased with the PD delay.
For maize plants with different PDs at two stages, namely, sowing–emergence and seeding, the average increasing rates of DST under mulching were 11.2% and 6.7%, respectively. Moreover, the warming effect gradually weakened with continuous GP progress, and the average increasing rates of DST under mulching were 2.3–3.0% during V6–harvest stage with different PDs. The results demonstrated an evident warming effect of film mulching in the prophase of maize.
3.1.2. Active Accumulated Temperature
The AATs (≥10 °C) in different treatments for various GPs of maize under mulched drip irrigation are shown in Table 2
The total AATs exhibited a decreasing trend with the delay in PD, decreasing under MD2–MD4 treatments by 73 °C, 226 °C, and 365 °C, respectively, compared with those in MD1.
The AATs at maize’s different GPs were determined based on daily temperature and stage duration. The average AAT required for maize emergence was approximately 208 °C, and the annual variation was caused by fluctuation in the occurrence of late spring cold.
The AATs at both vegetative and reproductive growth stages decreased with the postponement of PD. The AATs at these two stages under MD1–MD4 treatments were on the average 1494–1430 °C and 1327–1022 °C, respectively. Compared with those in MD1, AATs of the maize reproductive stage in MD2–MD4 decreased by 89 °C, 198 °C, and 305 °C, respectively, illustrating that PD particularly affected AATs of this stage and insufficient AATs at this stage were unfavorable for grain filling and dehydration.
3.2. Effect of Planting Date on Maize Growth Process
The durations of various maize GPs under different treatments are shown in Table 3
, and no statistically significant differences occurred between years for the average parameter values of all treatments.
Compared with that under NM-D2 treatment, the duration of total maize GP under MD2 decreased by 3 days, and that of sowing–VE period shortened by 4 days, whereas the differences in growth durations of middle–late stages were minimal.
For treatments under mulched drip irrigation, the duration of total maize GP decreased gradually with the delay in PD, and the differences among MD1–MD4 reached 5–23 days. The total GP duration for maize in both MD1 and MD2 exceeded 150 days, which guaranteed the physiological maturity for grain. Whereas, maize in MD3 and MD4 experienced early autumn freezing during later growth stages, resulting in evident shortening in GP duration (10–18 days) and failing to reach physiological maturity.
A significant difference reaching 2–8 days was found in sowing–VE duration among MD1–MD4 treatments. When sowing was performed on April 20, maize sowing–VE duration was highly unstable with a 9-day difference within 2 years because of the fluctuation in late spring cold. In 2015, maize had entered seedling stage as late spring cold occurred, whereas the occurrence of chilling injury during sowing–VE stage delayed emergence by 9 days in 2016. In MD2–MD4 treatments, the sowing–VE duration decreased gradually with PD delay with insignificant inter-annual differences.
The duration of VE–VT for maize showed a decreasing trend with the PD delay. A 12-day difference in this period appeared in MD1 treatment for 2 years because the seedlings underwent a long period of recovery after acquiring frostbite in 2015; by contrast, no chilling injury occurred during this period in 2016. The inter-annual differences of this period for 2 years were insignificant under MD2–MD4 treatments.
The effects of PD on the VT–Harvest duration were highly evident. MD1 and MD2 didn’t show significant difference during this period, but shortened by 4–8 days when PD delayed to May 12 or later (MD3 and MD4) because of early autumn freezing.
3.3. Effects of Planting Date on Maize Growth Indexes
Planting date significantly affected maize emergence rate, plant height, stem diameter, and single plant dry weight (DW) and its allocation proportions in each organ under mulched drip irrigation in varying degrees (Table 4
). No significant difference occurred in maize growth indexes under MD1
, but the plant height increased significantly and the stem diameter and DW per plant decreased considerably with the continuous delay in PD. And no appreciable differences existed in most growth indexes between years, other than plant height and single plant DW (with greater values in the first year).
The maize emergence rate under treatments of MD1, MD3 and MD4 did not appear to be markedly different, but that in MD2 decreased by 1.6–2.3%. The reason why maize in MD2 obtained a lower emergence rate was that the PD (May 2–3) was near the date of late spring cold, the average temperature during maize sowing–VE period was lower than that in other treatments.
A comparison of the distribution ratios of DW in nutritive and reproductive organs of maize under mulched drip irrigation showed that the proportions of DW in stem and leaves exhibited an upward trend with the delay in PD, whereas those in the ears presented a downward trend. No significant difference was found in single-plant DW and its distribution proportion in each organ between MD1 and MD2; however, in MD3 and MD4 treatments, single-plant DW decreased by 10.0–14.5%, and the proportions of DW in stem and leaves increased by 11.7–15.4% and 16.4–22.8%, respectively, whereas those in ears decreased by 8.6–6.4%. Results indicated that the delay in PD led to “slim” plants, which are not conducive to the translocation of dry matter to ears.
In the comparison of the maize growth indexes between treatments with film mulching and NM-D2, film mulching improved the maize emergence rate by 5.6–8.1% on average; the single-plant DW in MD1 and MD2 increased by 8.0–8.4%, while those in MD3 and MD4 decreased by 2.6–7.5%, compared with those in NM-D2.
3.4. Effects of Planting Date on Maize Yield Indexes
The maize yield indexes under different treatments are shown in Table 5
A comparison of the maize yield indexes with different PDs under mulched drip irrigation showed that the bald tip length and grain moisture content exhibited an increasing trend with a delay in PD, whereas the hundred-grain weight, grain number per ear (2016), yield, and total DM displayed a decreasing trend. The grain number per ear under MD2 in 2015 was the largest in terms of relatively lower emergence rate and large ear length. And no statistically evident differences existed in most yield indexes other than the bald tip length and grain number per ear between years, with smaller bald tip length and greater grain number in the first year.
No evident difference was found in the yield indexes between MD1 and MD2. Under MD3 and MD4, the bald tip length increased by 34.3% and 54.3%, the hundred-grain weight decreased by 4.3% and 9.1%, and grain number per ear decreased by 7.5% and 9.9%, respectively, compared with those in MD1 and MD2. The results illustrated that delayed sowing is unfavorable for the formation of yield components. Compared with those in MD1, under MD2, MD3, and MD4 treatments, DM decreased by 1.2%, 9.5%, and 13.7%, grain moisture content increased by 1.8%, 13.5%, and 17.9%, and yield (14% moisture content) decreased by 1.4%, 11.4%, and 17.3%, respectively. The findings indicated no significant difference in total biomass and economic yield of maize sowed on April 20 and May 2, whereas delayed PD to May 12 or later resulted in a significant yield reduction.
Compared with those yield indexes under non-plastic film mulching (NM-D2), the bald tip length and grain moisture of maize planted before May 3 (MD1 and MD2) averagely decreased by 20.5% and 3.8%, respectively, but the total DM and yield increased by 6.5% and 9.7%, respectively; while under MD3 and MD4 treatments, the bald tip length and grain moisture averagely increased by 9.1–22.7% and 8.0–12.3%, respectively, whereas the total DM and yield decreased by 1.5–8.0% and 2.2–6.6%, respectively. The results showed an apparent effect of film mulching on improving yield for the early sowing maize.
According to the significance F-test based on the mean dates of 2 years, illuminating that no evident differences were found in yield indexes between MD1
, whereas significant differences appeared in maize bald tip length, hundred-grain weight, and grain yield among treatments of MD2
, and MD4
. We use the regressions to present the relationships of maize bald tip length, hundred-grain weight, and grain yield with the days of PD delay (in Figure 6
, Figure 7
and Figure 8
). When the typical date of late spring cold (around May 1) was regarded as the starting point, an evident linear relationship existed between the grain yield indexes and the days of PD delay (MD2
). That is, for every 10 days of PD delay, bald tip length increased by 23.2%, whereas hundred-grain weight and grain yield decreased by 4.5% and 8.5%, respectively. According to the regression equation, it concluded that the yield penalty (4.2%) was less than 5.0% with a yield exceeding 15 Mg·ha−1
when PD delayed to May 8.
Therefore, obtaining a super high yield is possible under mulched drip irrigation when the medium-mature maize varieties with close-planting are sown from April 20 to May 8 in the area.