**5. General conclusion**

In Morocco, sugar beet is an important strategic crop. It is planted from September through June - July. Yield obtained by farmers, averaging 50 T/ha, is significantly below the request potential that would be 90 to 100 T/ha. Many factors contribute to low sugar beet production. Poor stand establishment, inadequate weed control, inadequate insect control and inadequate nitrogen fertilization are the main causes of low tonnage and poor quality sugar beet in Morocco.

This paper presents the main results of investigations and experiments conducted in Tadla region to improve the weed management program by identifying mains weed species encountered in sugar beet field, studiying the effect of weeds on sugar beet growth and estimating yield losses and determining the critical period of weed control and evaluating herbicide treatments.

One hundred twenty six (126) fields of sugar beet were surveyed by stratified sampling in Tadla region (Center of Morocco). In total, 144 weed species belonging to 30 botanical families were recorded. Six among them asteraceae, poaceae, fabaceae, brassicaceae, apiaceae and caryophyllaceae account 81 species (56,1% of total species). Dicotyledonous (81,9%), annuals (82,6%) and the Mediterranean floristic element (56,2%) were predominant and characterized the weed flora. The agronomic study made it possible to distinguish 24 species and volunteer wheat causing appreciable problems to the crop. Statistical analysis using soil-climatic factors allowed distinguishing four ecologic groups.

To determine the critical period of weed control in sugar beet, an experiment was conducted and consisting of 16 treatments. Weed free treatments included the removal of weeds at 4, 7, 9, 11, 13, 17 and 21 weeks after emergence (WAE) of sugar beet. In weed infested treatments, weeds were allowed to interfere with sugar beet crop 4, 7, 9, 11, 13, 17 and 21 weeks after emergence sugar beet crop. Weed infestation reduced root yield in all treatments. The presence of weeds during the entire growing season decreased root yield by 97.6 % and 68.9 % in 2003 and 2004, respectively. Based on 10 % permissible decrease in root yield, weeding should start from 4 WAE and 7 WAE in 2003 and 2004, respectively. For the given 10% root yield reduction, weed control should be continued until 15 WAE and 12 WAE in 2003 and 2004. The results show that the critical period begins earlier in 2003 and its duration is longer (77 days) comparatively to that observed in 2004 which is shorter (35 days) and begins relatively later.

A field experiment was conducted during two sugar beet growing seasons 2003/2004 and 2004/2005 in three locations to assess the efficacy of some herbicide treatments. These locations were chosen in order to have diversified weed flora and then have maximum information about herbicide activity spectrum. Concerning post sowing preemergence application treatments, Tramat combi (T1) provided good efficacy (90 % and more) and protected then the crop for a long period more than 2 months. This allowed to sugar beet to grow vigorously. The treatment controlled both dicotyledonous and monocotyledonous species. For post emergence applications, it was noted that when treatments were applied alone (not mixed), the efficacy was not satisfactory. Generally, herbicides (ethofumesate, metamitron, triflusulfuron methyl, phenmedipham, desmedioham and lenacil) achieved good efficacy when applied in tank mixtures than when applied individually alone because they are complementarily in eliminating maximum weed species. So, this must be taken in consideration in a weed chemical management program. These herbicide treatments allow to crop to grow within weed competitiveness nearly until the end of the critical period and are often followed by a mechanical cultivation or a hand hoeing.
