Effect of stocking rate on performance, diet selection and apparent total-tract digestibility among heifers grazing cover crops

Abstract Text:

Grazing cover crops can increase lands available for cattle production and reduce costs associated with winter feeding. Unfortunately, data are limited on stocking rates that allow optimal utilization of cover crops by cattle. We evaluated effects of stocking rate among heifers (BW = 573 ± 9.5 kg) grazing cover crops. Cover crop pasture (12.1 hectares) consisted of a mixture of annual ryegrass (Lolium multiflorum; 66.5%), radish (Raphanus sativus; 20%) and purple top turnip (Brassica rapa; 13.5%). Pasture was divided into 12 paddocks. Heifers were blocked by weight prior to initiation of the experiment and randomly assigned to 1 of 4 BW blocks. Stocking rate treatments were designed to obtain 45%, 55% or 65% forage utilization and were achieved by randomly assigning 3, 4 or 5 heifers within each BW block to paddocks. All paddocks in 3 BW blocks contained a ruminally cannulated heifer to facilitate measures of diet selection and heifers were allowed to graze for 48 d. Heifers were weighed on consecutive days at the beginning and end of the experiment and on d 9 to 22. Heifers were provided TiO₂ from d 14 to 23 and composite fecal samples (d 18 to 24) allowed estimates of fecal output. Estimates of DMI and diet digestibility were obtained from determination of fecal and diet acid detergent insoluble ash. Initial, intermediate and final diet samples were collected on d2, 24 and 46 by ruminal evacuation. Fecal and diet samples were analyzed for DM, OM, NDF and ADF. Reductions in stocking rate tended to increase intermediate (linear effect P = 0.06) and overall (linear effect P = 0.10) ADG. However, estimates of DMI tended (quadratic effect P = 0.07) to be less as stocking rate was reduced. Reduced stocking rate tended (Qqadratic effect P = 0.06) to increase diet DM; however, stocking rate had no impact (P ≥ 0.15) among diet OM, NDF and ADF. Estimates of DM and OM digestibility decreased (quadratic effect P < 0.01) with reduced stocking rate. Similarly, reduced stocking rate tended (linear effect P = 0.06) to reduce NDF digestibility, but stocking rate had no effect (P ≥ 0.23) among estimates of ADF digestibility.  These data indicate that reduced stocking rate among heifers grazing cover crops tends to increase performance. It is unclear why estimates of DMI and diet digestibility decreased with reduced stocking rate.

Keywords: Cattle, Cover Crops, Stocking rate