Effects of increasing rates of molasses supplementation and forage quality on the productivity of steers

Abstract

This study consisted of several experiments that investigated the feed intakes, and growth rates of steers fed low-quality Angleton grass (Dichanthium aristatum) supplemented with urea and sulfur (dry matter (DM) digestibility, 45%) and medium-quality Rhodes grass (Chloris gayana) (DM digestibility, 58%) when fed either alone or with molasses mixtures (MM, molasses plus nitrogen and phosphorus). Some digestion characteristics of the basal forage diets were also measured. Addition of MM at 1.2 times calculated maintenance energy requirements (M) was associated with about a four-fold increase in microbial crude protein production and a 42% decrease in neutral detergent fibre digestibility. In the first growth experiment, MM at 0M, 0.45M, 0.90M and 1.35M was offered and mostly consumed before forage was offered ad libitum. The initial mean weight of the steers was 240 kg. Consumed MM comprised 0%, 21%, 38% and 50% of the total DM intake for Angleton grass and 0%, 19%, 33% and 41% for Rhodes grass. The corresponding mean liveweight (LW) gains were 0.06, 0.31, 0.73 and 0.83 kg/day for steers fed Angleton grass and 0.61, 0.90, 1.02 and 1.13 kg/day for steers fed Rhodes grass. In a second experiment, MM was offered to steers (initially 370 kg) at the same time as forage. Offered MM comprised 0, 0.40, 0.80 and 1.20M of the calculated metabolisable energy (ME) requirements for maintenance. Consumed MM comprised 0%, 20%, 36% and 48% of the total DM intake for Angleton grass diets and 0%, 18%, 28% and 32% for Rhodes grass diets. The corresponding mean LW gains were –0.09, 0.36, 0.58 and 0.70 kg/day for Angleton grass diets and 0.63, 0.67, 0.80 and 0.94 kg/day for Rhodes grass diets. The method of presentation of the MM had only minor effects on the amounts and proportions of forage and the MM consumed and on the LW gains of the steers, expressed as g/kg0.75 LW per day to allow for differences in initial LW between experiments. Using modifications to the scheme of SCA (1990) to predict maintenance energy requirements, a value for efficiency of use of ME for growth was calculated from growth rates of steers fed Rhodes grass. When MM with an assumed content of 11 MJ ME/ kg DM was added to the forage diets, the calculated ME content of forage decreased with increased MM content, consistent with depression of fibre digestion. The relationships for the separate experiments indicated that at 50% MM in the diet, the magnitudes of the depression in the calculated ME content of the forages were 20–37% for Rhodes grass and 18–33% for Angleton grass.