Yoshida - 1981 - Fundamentals of Rice Crop Science

6.1. LOW YIELDS UNDER EXPERIMENTAL CONDITIONS IN THE
TROPICS UNTIL MID-1960s
Yields from experimental rice fields in the tropics have increased dramatically
with the advent of the new high yielding varieties. However, the farmers’ yields
still remain low.
When nitrogen is applied at rates exceeding 40 kg/ha, many traditional indica
varieties tiller profusely and grow excessively tall, lodge early, and yield less
(Chandler 1963). Data from 10 years’ work at Chinsurah station in West Bengal,
India, shows that rice responded to nitrogen up to 33.6 kg/ha (30 lb/acre). Beyond
that rate, yield decreased. When farmyard manure was added, yield decreases
from ammonium sulfate were even greater, no doubt simply because of the extra
amount of nitrogen contained in the manure. Lack of nitrogen responsiveness thus
hampers attempts to increase rice yield through increased application of nitrogen
(Table 6.1).
Two reasons for such performance of indica rices in the tropics were considered:
• The genetical association of undesirable plant characters with indica rices.
• The warm climate in the tropics.
Traditional tropical varieties are late maturing, photoperiod sensitive, and high
tillering. They have tall, weak, thick culms; long drooping, thin, pale-green
leaves; and relatively large lax panicles. These varieties are grown under subopti-