Feeding behaviour, diet digestibility, rumen fluid and metabolic parameters of beef cattle fed total mixed rations with a stepped substitution of wheat straw with maize silage

In order to verify the possibility of using maize silage (MS) as sole forage in beef cattle diets, four isofibrous diets were formulated with stepped substitution of wheat straw (WS) with MS. The four diets, MS0 (20WS:0MS as percentage of dry matter (DM) of the total diet), MS20 (10WS:20MS), MS35 (5WS:35MS) and MS50 (0WS:50MS) were fed as total mixed rations (TMR) to 4 Simmental bulls (384 ± 45 kg initial live weight) according to a latin square design with periods of 28 days. Diets were isocaloric and isonitrogenous and at the time of their formulation the stepped substitution of WS with MS allowed to progressively reduce the need for energy concentrates increasing the forage:concentrate ratio. The increasing inclusion of MS to replace WS affected also the particle size distribution of the diets with a significant increase of the percentage of particles retained by a 8 mm sieve (P b 0.001). However, dry matter intake (DMI) was not affected by these changes in the diet composition and physical characteristics. Bulls' average daily gain (ADG) was not significantly affected by the type of diet. Regardless of the different diets, the animals ate more than 70% of their daily dry matter in the first 8 h after feed delivery. Bulls took longer to consume the diet without silage (MS0) than any other diet while the time spent ruminating was similar across diets. Even when fed only a conventional MS as dietary roughage bulls did not select for the longest particles in the TMR (N 19 mm). Total tract apparent digestibility was influenced by diet type: lower values were recorded for the diets with a higher content of WS (MS0 and MS20) for DM, organic matter, crude protein, NDF, ADF (all Pb0.001), and to a lower extent for starch (Pb0.05). MS50 diet showed the highest values for all digestibility parameters. Rumen fluid parameters and blood indicators of acid– base status of bulls were similar across diets and they were at all times within safety range as the risk of acidosis is concerned. The results suggest that MS with a theoretical chopping length of 9 mm at harvest can be used as sole roughage source in beef cattle diets, without adverse effects on DMI, ADG, feeding behavior and health status of the animals.