Effects of increasing dietary zinc sulfate fed to gestating ewes: II. Milk somatic cell count, microbial populations, and fatty acid composition

Authors: PAGE, CHAD - Utah State University; KNUTH, RYAN - University Of Wyoming; Murphy, Thomas - Tom; RULE, DANIEL - University Of Wyoming; BISHA, BLEDAR - University Of Wyoming; Taylor, Joshua - Bret; STEWART, WHITNEY - University Of Wyoming

Submitted to: Applied Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/6/2022
Publication Date: 5/30/2022
Citation: Page, C.M., Knuth, R.M., Murphy, T.W., Rule, D.C., Bisha, B., Taylor, J.B., Stewart, W.C. 2022. Effects of increasing dietary zinc sulfate fed to gestating ewes: II. Milk somatic cell count, microbial populations, and fatty acid composition. Applied Animal Science. 38(3):285-295. https://doi.org/10.15232/aas.2022-02268.
DOI: https://doi.org/10.15232/aas.2022-02268

Interpretive Summary: Zinc is an essential trace mineral to maintain sheep health and productivity, but its concentration in forages is variable throughout the year. In extensively and semi-extensively managed sheep populations, forage zinc concentrations are lowest from late fall through winter which coincides with breeding, gestation, and early lactation. This deficit can be overcome by providing supplemental minerals to sheep, but it is estimated that 30 to 50% of extensively managed sheep operations fail to do this consistently. The impact of zinc supplementation above recommended levels on ewe udder health and milk composition in extensive and semi-extensive sheep production is not well understood. The objective of this research was to determine the effects of increased dietary zinc concentrations from mid to late gestation on subsequent indicators of udder health and milk fatty acid composition. Results indicated that feeding zinc above recommended levels during late gestation had little effect on altering indicators of udder health and milk component profiles in semi-extensively managed ewes. However, results suggest subclinical udder disease is prevalent and more so during early lactation than at weaning. Therefore, future research is warranted to investigate additional management methods that reduce bacterial load and target mechanisms by which pathogenic bacteria infect the udder.

Technical Abstract: Objective: The objective of the research was to evaluate the effects of increasing dietary Zn sulfate concentration for primiparous gestating ewes on subsequent milk SCC, intramammary microbial identifications, and fatty acid composition. Materials and Methods: Commercial white-face (WF; n = 27) and black-face (BF; n = 24) ewes (age ˜18 mo; BW = 87.48 ± 8.37 kg) were sorted into breed-type groups and within groups ranked by BW, and then, they were randomly divided into 3 dietary supplement treatment groups: CON (n = 13; 40 mg/kg Zn; ˜1 × NASEM recommendations), Zn500 (n = 21; 500 mg/kg Zn; ˜4 × NASEM recommendations), and Zn1000 (n = 17; 1,000 mg/kg Zn; ˜7 × NASEM recommendations). Treatments were administered in Zn-fortified pelleted alfalfa (0.45 kg/ewe per day) and fed from 87.5 ± 8.9 d of gestation until parturition. Milk traits collected at parturition (d 1 of lactation), ˜30 d of lactation, and lamb weaning (˜90 d of lactation) were assessed as repeated measures with fixed effects of treatment, breed type, and litter size. Results and Discussion: The treatment × breed type interaction affected ewe logSCC (P = 0.01), and within Zn500, BF had greater logSCC than WF ewes (5.90 ± 0.08 vs. 5.46 ± 0.08; P < 0.01). However, breed types did not differ between CON and Zn1000 treatments (P = 0.92). Ewe logSCC was greatest (P < 0.01) at weaning (6.03 ± 0.06), intermediate at parturition (5.72 ± 0.06; d 1), and least at d 30 of lactation (5.21 ± 0.06). Intramammary infections were common in milk samples collected at parturition (77%) and weaning (47%) based on culture-based microbial identifications. The most frequently identified species included Bacillus spp. and Staphylococcus spp. Black-face ewes had greater concentrations of C16:1 (1.78 mg/100 vs. 1.39 mg/100 mg of fatty acid per 100 mg of total fatty acids), C17:1 (0.86 mg/100 vs. 0.76 mg/100 mg of fatty acid per 100 mg of total fatty acids), and C20:4 (0.28 mg/100 vs. 0.24 mg/100 mg of fatty acid per 100 mg of total fatty acids; P = 0.04) than WF ewes. Implications and Applications: Including Zn in diets beyond NASEM recommendations from mid to late gestation had no effect on ewe milk SCC, microbial pathogens identified, or fatty acid composition. However, findings indicated there may be important breed differences in dietary Zn utilization and requirements affecting intramammary inflammation.