Non-structural carbohydrate content of the mixed ration affects both pasture and total intake of dairy cows via mechanisms explained by the hepatic oxidation theory

Abstract

Feed intake is the single most important factor driving milk yield in dairy systems. Partial mixed ration (PMR) feeding systems incorporate conserved forages and concentrates, combined in a mixed ration, into a grazing system. These systems have become the most common within the sub-tropical Queensland dairy industry; however, feed intake in PMR systems is highly variable (2012 Queensland dairy farm survey showed 12.2 to 25.0 kg dry matter (DM)/day, unpubl. data). Part of this variability is likely due to the two distinct feed types (mixed ration and pasture) having varying effects on intake. Ison et al. (2020a, 2020b) illustrated that pasture structure and allocation have significant effects on intake within sub-tropical PMR systems. Auldist et al. (2014) found that increasing the crude protein (CP) content of the mixed ration increased pasture intake for dairy cows in the temperate region of Australia. This current study was conducted to determine how the nutrient profile of the mixed ration affects pasture and total intake in the sub-tropical region. Three diets were formulated to be isoenergetic but vary in non-structural carbohydrates (NSC; starch and sugars), CP and fibre content. The differences in nutrient profiles were achieved by altering the proportion of wheat grain, canola meal and soyhulls within the mixed ration. Soyhulls were used as a non-forage fibre source so that the physical properties of the mixed ration were not affected. The experiment was conducted at the Gatton Research Dairy during spring in 2020. Thirty-six lactating Holstein-Friesian dairy cows were randomly allocated to one of the three experimental diets; High NSC; High CP or High Fibre. Targeted feed allocations were 15.5 kg DM/day of the mixed ration on a feed pad during
the day and 8.0 kg DM/day of lucerne (Medicago sativa L.) pasture grazed overnight.