Abstract
 
Background and Objectives
Khouzestan province with about 1000 hectares of seed clover is the most important region for seed production of berseem clover (Trifolium alexandrinum) in Iran. Extending the cultivation area of berseem clover (BC) is important due to the favorable effects of BC on soil fertility and the necessity of this region to a desirable crop rotation. The harvest date of summer crops such as corn leads to delay in optimum sowing date of BC. Several investigators in other areas have demonstrated that early sowing dates improved yield of BC. On the other hand, the most important factor in crop yields is water irrigation; it plays an important role in sustainable agriculture. Based on the existing literature, early sowing date reduces the number of irrigation events and increases water irrigation use efficiency (IWUE). Therefore, this experiment was conducted to evaluate the response of spring BC sowing date and last irrigation cutoff time on seed yield and IWUE.
 
Materials and Methods
This study was carried out in Safiabad Agricultural Research Center in two years (2014-15). Soil texture of the experimental field was silty clay loam, well drained without salinity (ECe=1.2ds m^-1), with 0.75 percent organic carbon. The statistical design of the experiment was split plot based on a randomized complete block design with four replications. The main plots were allocated to sowing date including 10^th Feb, 20^th Feb, 2^st Mar and 12^th Mar, and the sub plots were assigned to irrigation cutoff dates including 5^th Jun, 15^th Jun and 25^th Jun. Daily evaporation from class A evaporation pan of the Safiabad weather station (nearby the experimental field) was measured to calculate water irrigation depth, and BC evapotranspiration was calculated by applying a pan coefficient of 0.8. Irrigation interval was managed based on 80 mm accumulated evaporation from the pan evaporation. The measured traits included seed yield, IWUE, number of seeds per inflorescence (NSI), number of inflorescences per square meter (NI), days to flowering, straw production, total dry matter (TDM), harvest index (HI) and weighed a thousand seeds.
 
Results
There was a significant difference between the irrigation cutoff date for all traits except the number of days to flowering and HI. The results of the mean comparison showed that the, sowing date of 20^th Feb and irrigation cutoff in 15^th Jun, with seed yield (631 kg.ha^-1) and IWUE (0.72 kg.m^-3) was better than the other treatments. In this treatment, the plant growth period in the first sowing date (88.6 days), and the NI (623 numbers), and the NSI (37.1 numbers) was higher than the other treatments. The correlation between seed yield and other traits was positive and the stepwise regression analysis showed a positive and high effect of NI on seed yield in BC.
 
Discussion
The results of this trial confirm previous findings that BC early sowing dates, will had higher seed yield. Also, the results showed that delaying in irrigation cutoff is effective in decreasing IWUE, so that irrigation until 25
^th 
 June in comparison with 15
^th
 June, In spite of significant increasing in yield, decreased IWUE significantly.