The researchers studied different rice cultivars to see how efficiently they use nitrogen fertilizer to grow and produce grain. They found that increasing nitrogen levels helped increase grain yield, but only up to a certain point. Some rice varieties were better at using nitrogen efficiently than others, with high-efficiency varieties producing more grain with less nitrogen. The less efficient variety had fewer tillers, which meant it needed less carbon and nitrogen during grain filling, leading to lower efficiency. Overall, the study showed that choosing the right rice variety and nitrogen levels can help increase yield, reduce costs, and protect the environment.