| Sumario: | Increased organic matter and nitrogen intake of a low quality barley straw basal diet was achieved by supplementation with 15% and 30% of dry matter offered as Calliandra calothyrsus, Macrotyloma axillare or poultry manure. Urinary-N excretion for the basal diet (0.5 mg kg-1 liveweight (W) d-1 was similar to C. calothyrsus at 15% and 30% supplementation (1.3 and 0.8 mg kg-1 W d-1 respectively) and M. axillare at 15% and 30% supplementation (0.4 and 0.6 mg kg-1 W d-1 respectively). Due to small amounts of urinary-N excretion (approx. 1% of excreted N) no influence of tannins, derived from the C. calothyrsus supplemented diets, on the partitioning of excreted nitrogen between urine and faeces was observed. In contrast, the feeding of poultry manure, a supplement containing highly degradable N, resulted in larger excretions of excess rumen ammonia as N in the urine, 17.5 and 23.2 mg kg-1 W d-1 for 15% and 30% supplementation respectively. Different quality faeces (i.e. varying in total N, soluble N content and N bound to fibre), were produced by feeding cattle a barley straw basal diet alone or supplemented with 15% or 30% C. calothyrsus, M. axillare or poultry manure. In the one field season, no improvement in crop production was found after additions of faeces (equivalent to 25 kg N ha-1) derived from manipulation of diet quality or from the feed materials added directly to the soil (equivalent to 25 kg N ha-1) within the central highlands of Kenya. Although, as observed in the glasshouse pot experiment, the pattern of the release rate suggested that difference might not be seen in the first season after application but that net N release occurred after the first season. It therefore follows that benefit from these organic matter additions would not be expected in the first season but as mineralization continues after the first season then increased N availability may lead to increased plant growth and increased stover and grain DM yields in subsequent seasons
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