Resposta de vacas leiteiras em lactação à suplementação com monensina em dietas diferindo na capacidade acidogênica ruminal

Abstract

Monensin (MON) supplementation of dairy cows can interact with diet composition. The response of lactating dairy cows to MON supplementation of diets differing in ruminal acidogenic capacity was evaluated. Twenty-eight Holstein cows were individually fed a standard diet for 3 weeks and MON (300 mg/d) or control (CTL) for 9 weeks, in a covariate adjusted randomized block design. A low acidogenic diet (LOW. 25.8% starch) was offered from d 1 to 35 and a high acidogenic diet (HIGH. 30.0% starch) from d 36 to 63. Diet HIGH was formulated by replacing whole cottonseeds and finely ground corn by an iso-nitrogenous mixture of soybean meal and rehydrated and ensiled ground corn. Data were collected daily or after adaptation to LOW and HIGH and were analyzed as repeated measures. The mean square for cow nested within treatment was the error term for the treatment effect. Digestible organic matter intake was increased by HIGH (14.7 vs. 14.3 kg/d) and decreased by MON (14.2 vs. 15.2 kg/d) and milk yield was similar (32.3 kg/d). The MON reduced dry matter intake (DMI) and increased the ratios of milk to DMI, energy corrected milk to DMI, energy corrected milk to digestible organic matter intake and plasma glucose concentration only when HIGH was fed. The MON reduced the acetate to propionate ratio in ruminal fluid in both diets (2.43 vs. 1.81). The HIGH reduced ruminal pH. Plasma D-lactate concentration was increased by HIGH and decreased by MON. The HIGH increased total protozoa concentration in ruminal fluid. The MON reduced total tract neutral detergent fiber digestibility more in LOW (52.1 vs. 41.3% of intake) than in HIGH (57.0 vs. 52.0% of intake). Daily urinary allantoin excretion did not respond to MON, neither plasma urea-N at 0, 1.5, and 3 h post feeding. The MON induced lower degree of refusal of long feed particles and of preferential intake of short particles in the afternoon than CTL. Meal duration and interval was reduced by MON only when HIGH was fed. The HIGH tended to reduce jugular blood pH, increased the concentrations and yields of short-chain fatty acid (FA), and reduced the concentrations of long chain FA in milk. The MON did not affect the concentrations and yields of short, medium, and long chain FA, but reduced yields of odd and branched-chain FA (C15:0 iso plus C17:0 iso) in milk fat. The positive effect of MON on feed efficiency occurred only when diet acidogenic capacity was increased.