What is the meaning of "The Reapers" by Jean Toomer? Does it shed any additional light on the symbolism of the tractor in A Gathering of Old Men and its connection to the larger ideas of the novel?

Here, you have a three-part assignment that asks you to understand Toomer's work in light of his background.
The key biographical fact you want to discuss is Toomer's mixed race heritage and the way that has led to much of his work exploring the question of racial identity. Many of his works focus on the intersection of various cultures. For example, Candy, although white, regards Mathu almost as a father-figure, and the "Salt and Pepper" team also exemplify racial interdependence. 
In the poem, the initial quatrain focuses on black men sharpening scythes, while the conclusion of the poem focuses on the death of the rat. In the poem, the rat's death is obviously an accident. No one intended to kill it; it merely got in the way. The black men are just laborers doing their jobs. They don't own the fields and are not the ones who organized the harvest. What your instructor wants you to think about, given the juxtaposition of this and the novel, is how mechanization is an oppressive force that works together with racial injustice.
Chapter 9 of A Gathering of Old Men recounts a story in which a black man's mules prove more efficient than a Cajun's tractor. It is a power mower in the poem that kills the rat, something that would not happen with manual scythes.