Binary mixtures of birdsfoot trefoil and alfalfa with tall fescue: if and why they improve beef steer gains

High nitrogen (N) fertilizer costs and increased environmental stewartship have renewed interest in grass-legume pastures, and past research was not indicative of the productive, irrigated, rotational stocked pastures common in the western USA. This research compared herbage mass and nutritive value, steer weight gain, and economics of tall fescue-alfalfa [TF+ALF, Lolium arundinaceum (Schreb.) Darbysh., Medicago sativa L.], and tall fescue-birdsfoot trefoil (TF+BFT, Lotus corniculatus L.) mixtures, to tall fescue with (TF+N) and without N fertilizer (TF-N). Multivariate analysis examined which herbage variables primarily influenced steer performance. Pastures were established in Lewiston, Utah, USA, and rotationally stocked (28-day cycles) by Angus beef steers for 112 days in 2012 and 2013. Herbage samples were collected prior to each 7-day stocking period and analyzed for mass and nutritive value. Steers were weighed each 28-day rotation cycle and cumulative average daily gain (ADG) calculated. Steer performance differed (p < 0.05) amongst treatments with ADG greatest for TF+BFT (0.73 kg d-1), followed by TF+ALF (0.67 kg d-1), TF+N (0.61 kg d-1), and TF-N (0.40 kg d-1). Principal component analysis indicated that small differences in energy, digestibility, and fiber had the greatest influence on ADG. Other variables affecting ADG were legume content, herbage mass, and BFT content. Tall fescue-legume mixtures had 29 to 33% legume content, and more (p