Brunsvig B R, Smart A J, Bailey E A, Wright C L, Grings E E, Brake D W
J Anim Sci. 2017 Aug;95(8):3513-3522. doi: 10.2527/jas.2017.1563.
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, = 0.02), but increased stocking density decreased (linear, ≤ 0.01) intake of brassica compared with grass on d 48 (stocking density × time, < 0.01). Increased stocking density increased DM (quadratic, < 0.01), OM (quadratic, = 0.01), and NDF (quadratic, = 0.05) digestion, and stocking density tended to increase DMI (quadratic, = 0.07). Additionally, increased stocking density quadratically increased ( = 0.05) N retention but did not affect overall BW gains. Increased stocking density did, however, contribute to linearly decreased ( = 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 ( = 0.06) with greater amounts of time on pasture. Ruminal liquid and particulate fill and amounts of VFA were less (quadratic, ≤ 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.
在南达科他州燕麦谷物收获后放牧一年生冷季牧草，可能会提供一个增加可耕地高效利用的机会。然而，关于放牧密度对牛采食一年生冷季牧草时的日粮选择、养分消化、生产性能和氮保留的影响，数据有限。小母牛按初始体重（261±11.7千克）进行分组，并随机分配到12个围场（1.1公顷）中的1个，以放牧草和芸苔属植物的混合物48天。每个围场饲养3、4或5头小母牛，以实现每个放牧密度处理的4次重复。使用安装了瘤胃瘘管的小母牛来测量日粮和养分摄入量。在放牧2、24和46天后测量放牧密度对日粮和养分选择的影响，在实验开始、中间和结束时分别测量体重，分别为第1天和第2天、第22天和第23天、第47天和第48天体重的平均值。从第18天到23天收集干物质采食量（DMI）以及干物质（DM）、有机物质（OM）、中性洗涤纤维（NDF）和酸性洗涤纤维（ADF）消化率的测量数据。在第24天，放牧密度增加使芸苔属植物相对于草的摄入量增加（二次效应，P = 0.02），但在第48天，与草相比，放牧密度增加使芸苔属植物的摄入量减少（线性效应，P≤0.01）（放牧密度×时间，P<0.01）。放牧密度增加使DM（二次效应，P<0.01）、OM（二次效应，P = 0.01）和NDF（二次效应，P = 0.05）消化率提高，放牧密度有使DMI增加的趋势（二次效应，P = 0.07）。此外，放牧密度增加使氮保留呈二次增加（P = 0.05），但不影响总体体重增加。然而，放牧密度增加导致从放牧第1天到第22天体重增加呈线性下降（P = 0.05），但在实验后半期的体重增加大于从第1天到第22天的体重增加。在放牧第24天，瘤胃中乙酸：丙酸的浓度最低，瘤胃硝酸盐浓度随着在牧场上停留时间的增加呈线性下降趋势（P = 0.06）。随着在牧场上停留时间的增加，瘤胃液和颗粒物充盈度以及挥发性脂肪酸（VFA）的量减少（二次效应，P≤0.01）。显然，燕麦谷物收获后种植的芸苔属植物和草的二元混合物可以通过提供饲料资源来增加土地的高效利用。增加放牧密度可能有助于在以芸苔属植物 - 草为基础的牧场上放牧的牛更快地适应和采食芸苔属植物。
