Calcium and phosphorus play vital roles in the nutrition of ruminants, from DNA synthesis and intracellular communication to cell membrane fluidity and metabolic pathways (Goff, 2018; Underwood & Suttle, 1999). In turn, animals maintain a balance of minerals in their bodies that meets their nutritional needs by recognizing different internal states and discriminating among different nutrients (Provenza & Villalba, 2006). In this process, ruminants learn associations between the flavors of foods and their post-ingestive consequences (Provenza, 1995) such that foods that provide required amounts of Ca and P may be preferred, whereas those that supply excesses or deficits of minerals may be avoided (Provenza, 1996). This mechanism allows animals to self-select nutrients as a function of their individual and dynamic needs, instead of consuming a fixed amount of supplement designed for the average animal (Provenza et al., 2003). This behavior contributes to explain why lambs offered choices among different foods, typically improve productive and digestive performance (Askar et al., 2006), and show reductions in stress parameters (Catanese et al., 2013). Nevertheless, macrominerals in livestock production systems are in general provided in combinations and added to total mixed rations or presented in blocks. As described before, nutrient requirements are dynamic and depend on specific individual needs and physiological states. Thus, self-selection of minerals would account for such variability, allowing for an optimal state of nutrition and welfare within the herd. In support of

Beef products represent the fourth largest agricultural commodity in the state of California, valuing more than $3 billion from 2013 to 2015 (CDFA, 2016) and procure 90% of the income for the range livestock industry (FRAP, 2003). Forages found on California's coastal, desert, foothill, and mountain ranges are the basis of the state's beef cattle industry. Understanding their nutritional quality of these forages is important for their effective use (George et al., 2001a; Waterman et al., 2014). The objectives of this research were to investigate the nutritional characteristics, and in situ digestbilities in Angus beef cattle, of common California annual and perennial grasses: wild oats (Avena barbata and Avena fatua), soft chess (Bromus hordeaceous), filaree (Erodium botrys), Italian ryegrass (Lolium multiflorum), blue wildrye (Elymus glaucus), creeping wildrye (Leymus tritichoides), melic (Melica californica, Melica imperfecta, Melica torreyana), foothill needlegrass (Nasella lepida), purple needlegrass (Nasella pulchra). Nutritional composition as a percentage of dry matter (crude protein, CP; neutral detergent fiber, NDF; acid detergent fiber, ADF; and acid detergent lignin, ADL) and digestibilities were compared at four growth stages: late vegetative (LV), early reproductive (ER), late reproductive (LR), and dry (D). Plant samples were collected in San Luis Obispo County, CA. Crude protein concentrations decreased, and fiber concentrations increased, with maturity (P ≤ 0.05). Perennial grasses contained more NDF and ADF than annual grasses, across all growth stages (P ≤ 0.05). Annual grasses were significantly higher than perennials in dry matter digestibility (%DMD) at the 48 h incubation, when averaged across all growth stages (P ≤ 0.05); and at the LR and D stages, when averaged across all incubation periods (P ≤ 0.05). Within the annual grasses, %DMD was similar between ER, LR, and D stages. Within the perennial grasses, %DMD was similar between the LR and D stages (P ≤ 0.05).