A weft yarn is wound on a storage drum, and is withdrawn from the storage drum and inserted into a shed of a loom by a picking arrangement. A retaining pin is movable between first and second positions in which it respectively permits and obstructs removal of the yarn from the drum. A sensor is provided at a location spaced from the arrival end of the shed. A control arrangement responsive to the sensor effects movement of the retaining pin, and adjusts the point in time at which it moves the pin from its second to its first position in response to signals generated by the sensor during yarn insertion.