Summer annual forages are a potentially important supplement for summer grazing. The C₄ photosynthetic pathway allows increased mid-season growth relative to C₃ perennial pasture. Maximizing grazing availability is important to I) producers seeking to optimize the fatty acid content of their dairy products, and II) organic dairy producers that must satisfy the National Organic Program 30% minimum pasture rule. Although total fatty acid (FA) and polyunsaturated fatty acid (PUFA) content responses to nitrogen (N) fertility and harvest timing have been considered in C₃ forage species, to our knowledge no studies have enumerated the FA profiles of summer annual forage species. This experiment was conducted over two years (2013 and 2014) at two Vermont locations (loamy sand and silt loam, respectively) to assess the effects of N fertility and harvest timing on total FA and PUFA content of sudangrass (Sorghum × drumondii ‘Hayking’) and pearl millet (Pennisetum glaucum ‘Wonderleaf’). The experiment was a split-plot design, with harvest height whole plot treatments (60 cm vs. 90+ cm) and N fertility sub-plot treatments (39, 79, 118, and 157 kg N/ha). A measure of leafiness—the proportion of leaf blades relative to the entire plant as dry matter (DM)—was recorded, as most FA from vegetative forages are found in chloroplast thylakoid membranes. At both locations in 2013, harvests at 60 cm instead of 90+ cm resulted in higher total FA concentration (17.2 vs. 12.6 g/kg DM, P < 0.001) and a greater proportion of PUFA (62.3 vs. 57.7 g/100g total FA identified, P < 0.001). At the loamy sand location however, both total FA and PUFA effects of harvest height were present only in sudangrass (interaction effect, P < 0.0001). The loamy sand location was the only to show a sub-plot effect of increasing N fertility, with a total FA gain of only 1.9 g/kg DM from the lowest N level to the highest (P = 0.0478). Leafiness was positively correlated with PUFA content in sudangrass and pearl millet (r = 0.645 and 0.503, respectively). PUFA and total FA content of summer annual forages are primarily driven by maturity. Nitrogen fertility effects were confined to the low fertility soil location.