As shown in Table 2, weight gain, feed intake, and
feed/gain of male ducks responded quadratically to incremental
dietary arginine supplementation (P < 0.05),
and the low arginine diet significantly decreased feed
intake, greatly reduced growth, and increased feed/
gain (P < 0.05). The result is consistent with previous
studies in chickens by Kwak et al. (1999) and Labadan
et al. (2001). In the past, linear broken-line regression
was normally used to estimate arginine requirement of
broilers and ducks (Chen and Shen, 1979; Labadan et
al., 2001). However, the linear broken-line regression
yielded an underestimated nutrient requirement (