Relating Weed Size, Crop Soil Cover and Soil Moisture with Weed Harrowing Efficacy on Papaver rhoeas and Other Dicotyledoneous Weeds in Mediterranean Conditions

Abstract

ABSTRACT Twelve field trials were conducted in Catalonia (North-Eastern Spain) between 1998 and 2003 on different winter cereal fields highly-infested with Papaver rhoeas L. or, in two cases, with mixed dicotyledoneous weeds. Trials were conducted in semi-arid conditions in order to find out which are the real limitations of harrowing in those circumstances. A tine harrow was used in a single pass as the only weed control method at 4–6 km h−1 and on four occasions again later in post-emergence. Weed size, crop soil cover and climatic conditions were significantly correlated with initial efficacy so that small weeds, low crop soil cover allowing good harrowing and dry conditions enhanced weed control. Despite soil moisture not being significantly correlated with initial efficacy, it was significantly related to other factors like weed size, thus having an indirect influence on initial control. Initial efficacy ranged between 18 and 93% and was not correlated with final weed control at crop elongation, which was highly related to natural weed mortality. Efficacy increases in time were significant in four cases but decreases due to new germination flushes were not significant. Repeating harrowing later in post-emergence does not seem to be generally convenient as efficacy did in fact not decrease but increased only in one out of four cases. The results show how initial efficacy is highly dependent on small weeds, which are at the same time related to dry soils, dry climatic conditions and low crop soil cover. As harrowing was conducted as soon as possible in the trials, the real main limitation was thus climatic, i.e. having dry soils. Later natural weed mortality can increase significantly initial weed control but this did not occur in all cases. Thus, in Mediterranean conditions, harrowing should be conducted as early as possible as wet winter periods can also impede correct timing.