The numerical analysis of cometary dust tails (Fulle 1989) allows to obtain the mass loss rate, the size distribution, the ejection velocity and the orbital eccentricity of the dust grains ejected by the parent comets. All these physical quantities are necessary and sufficient to compute the comet mass contribution to the interplanetary dust cloud. We apply this method to three long period comets, namely C∕Bennett 1970II, C∕Bradfield 1987XXIX and C∕Liller 1988V, and obtain that each long period comet injects in bound orbits at least half of the total produced mass. When we consider that a typical long period comet can produce more than 1014 g of dust along each perihelion passage, we obtain that the considered long period comets alone injected a input mass rate of about 106 g s−1 of meteoroids in bound orbits during the last 20 years, a contribution which is very close to that from all short period comets.