This was observed in all four locations in year 1 and one of the
three locations in year 2. There was higher water content in the top
soil even at harvest time in year 1, and this improved water level
was likely to be a reason for increased crop establishment in experiments
where water logging was not an issue. Increased soil water
content with mulch was noted by others in mungbean in India
(Chaudhary et al., 1985) and peanuts in Vietnam (Ramakrishna
et al., 2006). The increased establishment may be also related to
lower temperature, as air temperature was already high at the time
of planting and mulch would have reduced soil temperature as
reported for maize and cowpea in Nigeria by Maurya and Lal (1981).
Rahman et al. (2005) also found about 50% increase in plant density
at 20 DAS in wheat in Bangladesh when mulched with rice straw,
as a result of improved soil temperature and moisture conditions.
While straw mulch improved crop establishment in most experiments
in the present study, the higher establishment alone may not
have fully explained the large increase in grain yield (mean increase
of 35%) as the increases in establishment were about 15–25% (from
72% to 83% in year 1, and 59% to 73% in Takeo in year 2). The experiment
in year 2 indicated that increased planting density by 25% had
no effect on grain yield in two of the three experiments conducted,
but had about 25% increase at the other location.