The response of irrigated, perennial ryegrass to fertilizer nitrogen was studied in four consecutive periods of the growing season by applying 0–350 kg N/ha to a fresh sward at the start of each period, and measuring both herbage dry matter and its content of nitrogen.
Responses in yield were highest in the first period, which ended at inflorescence emergence; in this period, both the percentage recovery of nitrogen and the extent of its utilization in producing dry matter were greater than in the later periods. Some damage to the sward was seen following the harvest of grass grown with the high levels of nitrogen in the first period. When the yields were 90% of the predicted maximum the nitrate-N content of the herbage ranged from 1000 to 2000 ppm, except in the first period when it was 200 ppm.
The response curves were used to calculate the nitrogen requirements of the grass which would maintain given incremental yield responses. To produce near-maximum yields, irrigated grass swards may require fertilizer nitrogen equivalent to 2 kg N/ha/day prior to inflorescence emergence, and up to 5 kg N/ha/day for the remainder of the growing season.
The apparent efficiency of conversion of the radiant energy, usable for photosynthesis, into plant energy averaged 3·2%; it did not vary greatly among the four periods.
The experimental results indicate the seasonal requirements of grass for fertilizer nitrogen and some of the implications for animal husbandry are discussed.