High nitrogen levels can lead to poor grain filling in rice plants by causing competition for nutrients. A study on super hybrid rice in China found that increasing nitrogen rates resulted in lower grain filling at the base of the panicle, where the spikelets are located. This was due to a decrease in fully filled spikelets and fertilized spikelet weight, as well as an increase in partially filled spikelets. Interestingly, the middle and top parts of the panicle were less affected by high nitrogen levels. Overall, the research suggests that high nitrogen input negatively impacts grain filling in rice plants, particularly in the lower part of the panicle.