Complete replacement of corn grain with crude glycerin for dairy cows

Abstract

The biodiesel process generates crude glycerin as a coproduct, which is primarily glycerol, but also includes varying amounts of other materials that may impact the value of crude glycerin as a feed for livestock. The response of dairy cows to the complete substitution of finely-ground mature corn with crude glycerin in diets containing a high content of non-forage fiber sources was evaluated. The goal was to maximize milk yield per unit of cultivated corn and to have a high replacement rate of corn starch by glycerol in the diet. Eighteen Holsteins (185±58 days in lactation) were housed in sand-bedded tie stalls and individually fed. Cows received a sequence of two treatments in a crossover design with 28-day periods. Treatments were: 1) control diet containing 14.8% ground corn (Control, CTL), or 2) a diet with complete substitution of ground corn with an isonitrogenous mixture of crude glycerin and soybean meal (Glycerin, GLY) at 12.3 and 2.7% of the ration dry matter (DM), respectively. Diets had (% of DM) 33.8% forage, 19.4% citrus pulp, and 8.6% whole cottonseeds. Crude glycerin contained 6.3% moisture, 76.2% glycerol, and 8,800 ppm of methanol, on an as-fed basis. No effect on DM intake (DMI, 16.7 kg/d) was observed, whereas GLY decreased milk yield from 23.4 to 21.3 kg/d. The daily secretion of lactose was lower (0.94 vs 1.05 kg/d) and there was a trend for total solids (2.57 vs 2.79 kg/d) to be lower for GLY than CTL. Plasma glucose concentration was decreased for cows fed the GLY diet (51.6 vs 58.3 mg/dL). Glycerin increased butyrate in rumen fluid (19.9 vs 15.6% of total volatile fatty acids) and decreased acetate (58.7 vs 64.6% of total volatile fatty acids) and the acetate to propionate ratio (2.79 vs 3.30). Total tract apparent OM digestibility was increased with GLY (74.1 vs 70.8% of intake). The ratios of milk yield to DMI (1.40 vs 1.30 kg milk/kg DMI) and milk energy secretion to digestible OM intake (DOMI) (1.47 vs 1.30 Mcal/kg) were decreased in GLY. There was no difference between treatments on rumen microbial yield, estimated by the daily excretion of urinary allantoin, and urinary allantoin per unit of DOMI. Eating time per unit of DMI was reduced for cows fed GLY (12.6 vs 14.2 min/kg DMI) and rumination rate did not differ between treatments (18.2 min/kg DMI). Overall, the substitution of ground corn with a crude glycerin and soybean meal mixture increased OM digestibility and decreased rumen acetate to propionate ratio, plasma glucose concentration, milk yield, and efficiencies of feed and energy utilization.