Effect of maize processing on diet selection in cows

Abstract

The effect of maize, processed by three different methods, on the diet selection process was studied in 12 fistulated dry cows in a randomized block design. The experimental period lasted 20 days and was repeated twice (eight animals/treatment). Treatments allowed selection of different processed maize grains by the animals: sugar cane and coarsely ground maize (CG), sugar cane and finely ground maize (FG) and sugar cane and steam-flaked maize (SF). Urea was used with sugar cane in all treatments in the same proportion, ensuring 10% of crude protein in the ingredients and avoiding the influence of nitrogen (crude protein) on the selection process. The selection process was evaluated by comparing (1) the chemical composition (total digestible nutrients, neutral-detergent fibre) of the diets selected by the animals and (2) ruminal fermentation variables (pH, volatile fatty acids-VFA, and ammoniacal nitrogen). Maize intake by cows in the SF treatment was 45.08 and 42.08% lower than the CG and FG treatments, respectively. This resulted in a 24.52 and 22.58% lower total dry matter intake (DMI), a 29.83 and 27.59% lower total digestible nutrient (TDN) intake in kg, a 9.29 and 8.62% lower TDN intake in % and a 45.70 and 47.19% higher neutral detergent fibre (NDF) intake in % by cows fed SF compared to CG and FG, respectively. However, degradable dry matter intake (kg) and degradable starch intake (kg and %) did not differ among treatments. These data suggest that maize-processing influenced diet selection, probably due to different energy content and availability in processed grains used, and that animals were able to recognize differences among maize degradabilities, which are not determinable by usual chemical analyses. Regarding ruminal parameters, total VFA, acetate and butyrate concentration, pH and ammonia nitrogen did not differ among treatments, indicating that animals made practical choices with the objective of maintaining an optimum ruminal environment. However, differences in propionate concentration and acetate:propionate ratio were observed, probably due to lower intake of steam-flaked maize in the SF treatment.