A basal mixed ration supplying 36 mg of Zn/kg of dry matter (DM) was supplemented with 1 of 4 concentrates differing in level and form of dietary Zn. The concentrates were fed at 2 kg/cow per day and contained 300 mg of Zn/kg (to supply the total recommended level, according to NRC (2001); R) or 60 mg of Zn/kg (to supply 0.66 of the total recommended level; L), either supplemented as ZnO (I) or organically chelated Zn (O). Forty-four Holstein-Friesian dairy cows (12 primiparous and 32 multiparous), on average 31 d (SD +/- 11.4) into lactation, were allocated to 1 of the 4 treatments. All cows remained on the treatment for 14 wk. The data was analyzed by ANOVA as a 2 x 2 factorial design. Dry matter intake averaged 23.5 kg/d and did not differ between treatments. Cows supplemented with organically chelated Zn at the recommended level of inclusion (RO) had a higher milk yield (37.6 kg/d) than those fed inorganic Zn at the recommended level (RI; 35.2 kg/d) or organically chelated Zn at the low level (LO; 35.2 kg/d), but was not different from those fed inorganic Zn at the low level (LI; 36.0 kg/d). Milk composition was unaffected by dietary treatment. Animals that received the low level of Zn (LI and LO) had higher somatic cell counts [3.97 and 3.93 versus 4.35 and 4.55 (log(e)) for RI, RO, LI, and LO, respectively] and milk amyloid A levels than those receiving the recommended levels (RO and RI). There was no effect of treatment on body condition score, body weight, or locomotion score. Hoof hardness improved over the duration of the study but there were no differences between treatments. Similarly, blood plasma mineral levels for Zn, Cu, Mo, and Fe were not affected by treatment, whereas there was a trend for increased ceruloplasmin levels in cows receiving the recommended compared with the low level of Zn, but there was no effect of mineral form. There was also no effect of treatment on superoxide dismutase activity or blood hematology. It is concluded that supplementing Zn at the recommended level reduced somatic cell counts and milk amyloid A levels, whereas supplementation in an organic form at the recommended level also increased milk yield.

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The teat canal in dairy cows is lined with keratin, which is considered to be a natural physical and chemical barrier involved in providing resistance to bacteria gaining entry into the mammary gland. Keratin is partially removed during milking therefore requiring regeneration. It has been estimated that a cow needs to regenerate 1.3 mg per teat between each milking to replace the amount lost (Capuco et al., 1992). Zinc is important in the process of keratinisation and previous studies have reported an 2.4 kg/d increase in milk yield when supplemented with Zn in an organic form at the recommended level and an increase in somatic cell counts (SCC) in cattle receiving 0.66 of the recommended level of dietary Zn (Atkin et al., 2007), although mechanisms for this observation are unclear. The objective of this study was to establish the effect of level and form of Zn (organically chelated (Bioplex ZnTM; Alltech Inc., Nicholasville, USA) vs. inorganic (ZnO)) and no supplemental Zn on dairy cow performance and keratin production in the teat canal.