Effect of harvest date and nitrogen fertilization rate on the nutritive value of amaranth forage (Amaranthus hypochondriacus)

This study was conducted to assess effects of harvest date (i.e., 40 and 60 d after planting) and N fertilization rate (i.e., 120, 180, 240 kg N/ha) on the nutritive value of amaranth forage (Amaranthus hypochondriacus) using a factorial experiment with a randomized complete block design. The content of dry matter (DM), crude protein (CP), true protein (TP), ether extract (EE), water soluble carbohydrates (WSC), ash-free neutral detergent fiber (NDFom), ash-free acid detergent fiber (ADFom), lignin(sa), ash, Ca, P, Na, K, oxalic acid and nitrate were determined. Soluble CP (SP) and protein fractions non-protein N (A), true protein rapidly degraded in the rumen (B₁), true protein degraded in the rumen at a moderate rate (B₂), true protein associated with the cell wall and slowly degraded in the rumen (B₃) and acid detergent insoluble CP (C) were measured according to the Cornell Net Carbohydrate and Protein System. In vitro gas production (IVGP), OM disappearance (OMD) and NDFom disappearance (NDFD) were determined using a gas production technique. Results showed that the later harvest date increased (P<0.05) DM, EE, WSC, NDFom, ADFom, lignin(sa), B₃ and C; while CP, TP, ash, Ca, P, K, SP, A, B₁, B₂, nitrate, total and soluble oxalic acid, IVGP, b (i.e., gas production from the insoluble fermentable fractions at 120 h), c (i.e., rate of gas production during incubation), OMD and NDFD decreased (P<0.05). With increasing N fertilization rate, CP, TP, EE, P, nitrate, oxalic acid, SP, A, b, OMD and NDFD increased (P<0.05), however B₂ declined (P<0.05). Increasing N fertilization increased yield, CP concentration and nutrient digestibility. At 40 d after planting use of amaranth forage as a ruminant feed is limited due to its high nitrate content. However, at 60 d, although a depression in digestibility and CP content occurred, this forage has the potential as a ruminant feed due to the much lower nitrate levels.