What is the highlight event in Jane Smiley's novel MOO?
Moo is, in essence, a novel against capitalism. As such, the reader can be sure that what you call the "highlight event" is when Earl Butz escapes from Old Meats as the building is demolished suddenly. This escape of the primary hog takes the cake.
The general summary of Moo involves Moo University which concentrates on agriculture (especially in regards to art and science). Academia at Moo University is rife with division and it comes to a climax when the Chairman of the Horticulture Department decides to fantasize about killing Dean Walker. While other professors are found eavesdropping and participating in plots, one student simply takes care of his pig, Earl Butz. Such is the story of the dark world of agricultural academia.
Apart from that general summary, there are other important events, too. (I mention them because the "highlight event" is really an opinion question, so feel free to choose one of the others. For example, when Stroop suffers an attack, her important equipment plans are left unfinished. This is a threat to Moo University funding. The budget cut proposed by Early is another important even (almost leading to the close of the university). Later, Gift reports that there is gold in Costa Rica, proposing a mining idea there (which becomes the choice of the professors). Further, racial tension escalates when Mary is called a vile name which sends her into social withdrawal.
In conclusion, though, we have to wait until chapter 64 to find the heat of Old Meats in the escape of Earl Butz. It is this moment when the hog becomes a prominent symbol of a college deep in crisis mode. Earl Butz, himself a pig, is a very important project at Moo University. The irony is, he is top secret and Moo's top priority. Agricultural academia, therefore, becomes the subject of much ridicule.