The automotive upholstery industry, which processes bovine leathers, has struggled with vast amounts of solid waste, of which the majority are fractions, such as shavings and splits and offcuts, both containing chromium (Cr) and free of Cr. In this work, a novel leather waste-to-fertiliser approach has been tested: four lightly processed (incubation with effective microorganisms) and three aggressively processed (using pyrolysis or hydrolysis processes) tanned leather waste fractions were used. They were applied as organic nitrogen-based fertilisers in two separate glasshouse experiments, consisting of four subsequent harvests, under spring–autumn conditions in northern Poland. The Cr stressing effect caused by bovine shavings containing Cr and splits and offcuts containing Cr stimulated an increase in ryegrass growth after 30 days, providing twice the dry matter yield than bovine shavings free of Cr and splits and offcuts free of Cr, then (after 90 and 120 days) it inhibited growth. The Monod kinetics model was developed using the best fitting procedure (R2 > 0.94) for the efficiency of internal N use in ryegrass for the lightly processed fractions of tanned leather-waste fractions and each harvest. In the case of the processed leather waste-fractions, the relation was quasi linear, evidencing less growth inhibition and no over-fertilisation effects. The acid hydrolysate of bovine shavings containing Cr showed the best performance, reaching a relative agronomic effectiveness (RAE, N utilization-based) equal to 89%, 95%, 19%, and 38% for 30, 60, 90, and 120 days, respectively, and 71% in total. For the lightly processed fractions, it was lower than 12%. The future perspective should focus on steps including scaling up the tests to field conditions; transportation, storage, and application methods; effectiveness for other crops; usage in crop rotation systems; environmental impact assessment; production carbon footprint; commercial potential; and optimizing the production processes.