Effects of feeding sulfate trace minerals above recommendations on nutrient digestibility, rumen fermentation, lactational performance, and trace mineral excretion in dairy cows

Most trace minerals (TM) are fed above dairy cow requirements in commercial herds but their fate and effects on dairy cows have not been well documented. In this study, we evaluated the effects of feeding short-term sulfate TM above recommendations on apparent total-tract digestibility of nutrients, rumen fermentation characteristics, serum concentrations, milk yield and composition as well as milk, fecal, and urinary TM excretion in mid-lactation dairy cows. Eight multiparous Holstein cows [average body weight: 684 (SD: 29) kg at 82 (SD: 10) days in milk] in a quadruple 2 × 2 cross-over design were fed a basal diet, differing in sulfate TM supplement concentrations, to provide either 0.11, 17, and 63 (control; CON) or 0.95, 114, and 123 (high trace minerals; HTM) mg of dietary Co, Mn, and Zn/kg of dry matter, respectively. Each experimental period had a 21-d adaptation to the diet, followed by a 10-d sample collection period. Feed ingredients and total feces and urine were collected during 4 consecutive d and rumen fluid was collected 0, 1, 2, 4, and 6 h relative to feeding. Milk yield was recorded daily and milk samples were collected on 4 consecutive milkings. Ingestion of Co, Mn, and Zn was higher for HTM compared to CON group by 216, 233, and 93%, respectively. Dry matter intake averaged 25.0 (SE = 0.6) kg/d, and apparent total-tract digestibility of major nutrients was similar between treatments. There was no measurable effect of HTM on ruminal pH, major volatile fatty acids, and protozoa counts. Isovalerate molar proportion was 9.4% greater for HTM compared to CON group. Neither milk yield (43.5 kg/d; SE = 0.8) nor milk fat and protein concentrations differed between treatments. Milk urea nitrogen concentration was significantly higher for HTM (11.7 mg/dL) compared to CON group (9.7 mg/dL; SE = 0.7). Fecal excretion of Co, Mn, and Zn increased by 223, 198, and 75%, respectively, for HTM compared to CON group. Urinary excretions of TM were marginal compared to feces, and only urinary Co and Mn were significantly higher for HTM than CON cows as similarly obtained for serum Co and Mn concentrations. Milk TM yields were not modified by treatments. In summary, short-term dietary sulfate TM supply over the recommendation did not improve cow performance but significantly increased fecal TM excretion, which could have impacts on TM accumulation in soils where manure is applied and could potentially result in leaching into nearby watersheds. Further studies are needed to evaluate the impact of high fecal TM excretion on the environment using the One Health approach. Moreover, the impacts of TM oversupply on milk production and cow health should be evaluated by long-term experiments.目前商业牛群中添加的多数微量矿物质（trace minerals, TM）剂量均高于泌乳奶牛的营养需求，但其在奶牛体内的代谢过程及对奶牛的影响尚未得到充分研究。本研究评估了短期饲喂超推荐剂量的硫酸盐型微量矿物质，对泌乳中期奶牛的表观全消化道营养素消化率、瘤胃发酵特性、血清矿物质浓度、产奶量及乳成分，以及乳、粪便和尿液中微量矿物质排泄量的影响。试验采用四组2×2交叉设计，选取8头经产荷斯坦奶牛（初始体重684±29 kg，泌乳天数82±10 d），饲喂两种硫酸盐型微量矿物质补充剂浓度不同的基础日粮，分别提供每千克干物质0.11、17、63 mg的钴（Co）、锰（Mn）和锌（Zn）（对照组，CON），或0.95、114、123 mg的上述矿物质（高微量矿物质组，HTM）。每个试验期包含21 d的日粮适应阶段，随后为10 d的样品采集阶段。连续4 d采集饲料原料、总粪便及尿液，于饲喂后0、1、2、4、6 h采集瘤胃液。每日记录产奶量，并连续4次挤奶采集乳样。HTM组的钴、锰、锌摄入量分别较CON组高出216%、233%和93%。试验奶牛的平均干物质采食量为25.0 kg/d（标准误SE=0.6），两组主要营养素的表观全消化道消化率无显著差异。HTM对瘤胃pH、主要挥发性脂肪酸浓度及原虫计数均无可检测到的影响，但HTM组的异戊酸摩尔比例较CON组高9.4%。两组的产奶量（43.5 kg/d，SE=0.8）、乳脂及乳蛋白浓度均无显著差异，但HTM组的乳尿素氮浓度（11.7 mg/dL）显著高于CON组（9.7 mg/dL，SE=0.7）。与CON组相比，HTM组的粪便钴、锰、锌排泄量分别增加223%、198%和75%。尿液中微量矿物质的排泄量远低于粪便，仅尿液钴和锰的排泄量在HTM组显著升高，这与血清钴和锰浓度的变化趋势一致。两组的乳中微量矿物质产出量无显著差异。综上，短期饲喂超推荐剂量的硫酸盐型微量矿物质并未改善奶牛生产性能，但显著增加了粪便中微量矿物质的排泄量，这可能导致施用粪肥的土壤中微量矿物质积累，并有可能渗入周边流域。未来需采用一体化健康（One Health）方法开展研究，评估粪便高微量矿物质排泄量对环境的影响。此外，还需通过长期试验评估微量矿物质过量添加对奶牛生产性能及健康的影响。