Estudio del efecto del tipo de forraje y el nivel de proteína bruta sobre los parámetros productivos y el perfil metabolómico en terneros frisones en cebo intensivo

Nowadays, beef cattle feedlot fattening faces different challenges. On the one hand, the research on new protein sources for animal nutrition would reduce the system’s dependence on soya. On the other hand, the dietary protein level optimization would reduce animals’ emission of pollutant compounds. For these reasons, the aim of this work was to study the impact on productive performance and metabolic profile of a crude protein restriction and a forage intake increase, in growing Holstein calves intensively reared. Firstly, the response of a dietary protein level decrease below the standards proposed by FEDNA (2008) was evaluated. Secondly, the forage intake increase reached by improving its quality was tested. Eighty Holstein calves in growing period were assigned to four treatments: 1-BP: low protein concentrate (10 per cent crude protein) plus barley straw; 2-CTR: standard concentrate (12 per cent crude protein) plus barley straw; 3-AV: standard concentrate (12 per cent crude protein) plus oat silage and 4-VZ: standard concentrate (12 per cent crude protein) plus vetch silage. Average daily gain and concentrate intake were automatically registered in farm facilities. A double marker system was used to determine the forage intake: chromium oxide (Cr2O3) as external marker and acid insoluble ashes as internal marker. Thus, nutrient intake, excretion and digestibility were calculated. Nitrogen balance was estimated from nitrogen intake and nitrogen urinary and fecal excretions. Rumen fermentation was characterized by determining ammonia nitrogen and volatile fatty acid concentrations and pH. Rumen bacterial and archaeal community composition was analyzed by taxonomic profiling of 16S ribosomal RNA variable regions. Ruminal protozoa cells were counted using an optical microscope. Microbial protein synthesis in rumen was calculated from purine derivatives urinary excretion. Finally, metabolic profile was determined from urine and plasma samples by liquid chromatography coupled with mass spectrometry. The results showed that, at the beginning of the growing phase, crude protein restriction reduced average daily gain. Dry matter, organic matter and nitrogen intakes were reduced, and also their apparent digestibility. Nitrogen urinary loss was lessened and nitrogen retention and accretion remained unchanged but tended to be lower. At the end of the growing phase, differences in animals’ productive performance disappeared and calves’ body weights became equal between treatments. The replacement of barley straw for high quality forages reduced concentrate intake of the whole growing phase. Productive performance of calves eating vetch silage was lessened, but not the one of those calves eating oat silage. For this reason, we considered oat silage as the best forage to use in combination with a standard commercial concentrate. Both crude protein restriction and forage intake increase raised rumen microbial biodiversity levels, thus only 34 per cent of detected operational taxonomic units were common between animals and experimental treatments. Dietary protein restriction led to an increase in proteolytic bacterial communities (i.e. Prevotellaceae and Olsenella), which may constitute a rumen microbiome strategy to cope with protein shortage. Forage intake increase achieved in animals consuming vetch silage led to an increase in fibrolytic bacterial communities (i.e. Fibrobacteres and Spirochaetes) and in ruminal archaeal population. Moreover, inclusion of vetch silage in calves’ diet allowed protozoal community to establish in rumen. New metabolites, which were discriminant between experimental treatments, became present because of crude protein restriction.