Effect of stocking density on performance, diet selection, total-tract digestion, and nitrogen balance among heifers grazing cool-season annual forages

Abstract

Grazing annual cool-season forages after oat grain harvest in South Dakota may allow an opportunity to increase efficient use of tillable land. However, data are limited regarding effects of stocking density on diet selection, nutrient digestion, performance, and N retention by cattle grazing annual cool-season forage. Heifers were blocked by initial BW (261 ± 11.7 kg) and randomly assigned to 1 of 12 paddocks (1.1 ha) to graze a mixture of grass and brassica for 48 d. Each paddock contained 3, 4, or 5 heifers to achieve 4 replicates of each stocking density treatment. Ruminally cannulated heifers were used to measure diet and nutrient intake. Effects of stocking density on diet and nutrient selection were measured after 2, 24, and 46 d of grazing, and BW was measured at the beginning, middle, and end of the experiment as the average of d 1 and 2, d 22 and 23, and d 47 and 48 BW, respectively. Measures of DMI and DM, OM, NDF, and ADF digestion were collected from d 18 to 23. Increased stocking density increased intake of brassica relative to grass on d 24 (quadratic, P = 0.02), but increased stocking density decreased (linear, P ≤ 0.01) intake of brassica compared with grass on d 48 (stocking density × time, P < 0.01). Increased stocking density increased DM (quadratic, P < 0.01), OM (quadratic, P = 0.01), and NDF (quadratic, P = 0.05) digestion, and stocking density tended to increase DMI (quadratic, P = 0.07). Additionally, increased stocking density quadratically increased (P = 0.05) N retention but did not affect overall BW gains. Increased stocking density did, however, contribute to linearly decreased (P = 0.05) BW gains from d 1 to 22 of grazing, but BW gains during the latter half of the experiment were greater than BW gains from d 1 to 22. Ruminal concentration of acetate:propionate was least on d 24 of grazing, and ruminal nitrate concentration tended to linearly decrease (P = 0.06) with greater amounts of time on pasture. Ruminal liquid and particulate fill and amounts of VFA were less (quadratic, P ≤ 0.01) with greater amounts of time on pasture. Apparently, binary mixtures of brassica and grass planted after oat grain harvest can provide an opportunity to increase efficient use of land by providing forage resources. Increased stocking density may facilitate a more rapid adaptation to and intake of brassica among cattle grazing brassica–grass–based pastures.