Summer forages under irrigation. 3. The effect of nitrogen fertilizer on the growth, mineral composition and digestibility of a sorghum cross Sudangrass hybrid and Japanese barnyard millet

Abstract

Sorghum bicolor x S. sudanense hybrid cv. Sudax ST6 and Echinochloa utilis cv. Shirohie were grown under irrigation at Trangie, New South Wales. Six rates of nitrogen (N) fertilizer were applied at sowing: 0, 25, 50, 120, 300, 450 kg N/ha. Height and dry matter yield were recorded at head emergence, together with the leaf hydrocyanic acid (HCN) potential, and leaf and stem dry matter digestibility, nitrogen, sulfur and sodium contents. Both the sorghum hybrid and the millet reached maximum forage yields (16.1 and 7.6 t/ha, respectively) at 350 kg N/ha. However, the tall-growing forage sorghum produced more dry matter than millet and consequently was more responsive to fertilizer nitrogen. In sorghum the leaf HCN potential increased linearly in response to N (8.5 x 10-4 %/kg N.ha). Since there was no compensating increase in the sulfur content (0.072%), and, assuming a requirement of 1.2 g S/g of HCN detoxified, sorghum lacked sufficient sulfur to detoxify the HCN potentially released in the rumen from sorghum receiving 150-200 kg N/ha. At this rate of nitrogen application, sorghum was producing only two-thirds of its forage yield potential. The sodium content of sorghum (0.013%) was not affected by nitrogen. In millet the nitrogen and sodium contents increased as applied nitrogen promoted growth. Plant sulfur was diluted initially but the nitrogen: sulfur ratio increased steadily (0.0067 units/kg N.ha). In contrast to sorghum, therefore, nitrogen fertilizer can be used to increase the dry matter yield of millet without jeopardizing forage quality and hence animal production.