A symphony written by a composer who grew up on a southern Minnesota dairy farm will make its world debut Saturday night in Des Moines. The working farm itself is a kind of symphony, says Steve Heitzeg, who also sees his composing as a type of farming, planting notes like seeds, hoping they’ll grow to sustain people.He says the musical staff is similar to the rows of crops and the noteheads are like the seeds. Heitzeg calls “Symphony to the Prairie Farm” a tribute to the vanishing way of life. The 20-minute work in four movements depicts the four seasons on a Midwestern farm. Heitzeg says the symphony will be playing some non-traditional instruments:The percussion section of the Des Moines Symphony will play buffalo bones, field stones, cow bells, and pieces of farm implements. Heitzeg, who lives in St. Paul, says they’ll also squeeze squeaky toys to create the illusion of prairie dog barks as a protest against that creature’s destruction. The symphony debuts Saturday at 8 p.m. at the Civic Center of Greater Des Moines.