Results and discussion

After 90 d of experimentation, the mean somatic cell count (SCC) in the control group increased to 388.1 ± 19.1 × 10³ cells/ml from 311 ± 20.9 × 10³ cells/ml at day 0. During the same period, the GI group had a drop in milk SCC from 322.1 ± 25.6 × 10³ cells/ml to 256.9 ± 20 × 10³ cells/ml. Similarly, SCC dropped from 345.2 ± 19.7 × 10³ cells/ml to 265 ± 19.3 × 10³ cells/ml in GL group. Essential oils showed comparable efficacy to both treatment groups, resulting in a significant decrease in SCC from 326.3 ± 22.1 × 10³ cells/ml at 0 d to 231.6 ± 21.6 × 10³ cells/ml after 90 d trial (Fig. 1). Lower levels of somatic cell count in the treatment groups clearly indicate that post-milking teat dipping was effective in preventing the invasion of pathogenic microorganisms into the mammary gland and thereby resulted in improving udder health. Increased SCC readings indicate udder illness, lower milk production, changes in milk composition, higher production costs and lower profit. Through the teat canal, bacteria enter the mammary gland and multiply in the milk. The mammary epithelium's ability to function is directly impacted by bacterial toxins, enzymes and cell wall components. Additionally, they promote the production of several inflammatory mediators, including neutrophils and lymphocytes, which lead to a rise in SCC and oedema from increased vascular permeability and vasodilation (Sharma et al., Reference Sharma, Singh and Bhadwal2011). According to our data, the antibacterial, anti-inflammatory and immunological modulatory properties of essential oils influenced the udder's defence system and improved udder health, explaining the decline in the number of SCC in the GEO group. Teat disinfectants offer a broad spectrum of efficacy and kill swiftly while leaving a persistent film on the teat skin that forms a physical and chemical barrier that provides long-term protection. When comparing the treatment group to the control group which received no teat dip (GC), the results of the current study on SCC are consistent with Kamal and Bayoumi (Reference Kamal and Bayoumi2015) findings, who used 0.5 percent iodine. Similar findings were also reported by Alawneh et al. (Reference Alawneh, James, Phillips, Fraser, Jury, Soust and Olchowy2020) who observed that somatic cell counts for cows administered an iodine- or lactobacillus-based solution after milking followed a similar trend.

Figure 1. Average values (±se) of somatic cell counts (SCC), California mastitis test (CMT) scores and electrical conductivity (EC) in control and treated animals.

The milk CMT scores of the groups that received post-milking teat dip based on essential oils, iodine and lactic acid did not differ statistically. However, the CMT scores of all treatment groups were significantly lower than that of the control group commencing on the 45th day of sampling (Fig. 1). The fact that the CMT response is a measure of leucocyte counts (SCC) and that SCC was significantly higher in the control group than in the treatment group, may help to explain the increased average CMT score in treatment group. Our findings are supported by those of Kamal and Bayoumi (Reference Kamal and Bayoumi2015), who showed that a teat-dipped group had a lower CMT score than a non-teat-dipped group. Comparable to our findings Singh et al. (Reference Singh, Sharma and Singh2018) found a significant decrease in CMT by using the combination of povidone-iodine and glycerin in a ratio of 3:1.

The first two samples (0 and 15 d) exhibited no significant variation in milk electrical conductivity, whereas a significant (P < 0.05) difference between GC and each of GI, GL and GEO was apparent at 45 and 90 d of sampling (Fig. 1). One possible explanation for this trend in EC is that it may be the result of the control group's higher infection frequency with subsequent sampling. The active Na, Cl, and K pumping mechanisms of the mammary gland as well as tight junctions are disrupted in cows exposed to intramammary infection, causing these ions to escape into the milk leading to increased EC (Norberg et al., Reference Norberg, Hogeveen, Korsgaard, Friggens, Sloth and Løvendahl2004).

When the group milk yields were compared, the treatment groups produced significantly higher milk than the control group (online Supplementary Table S1). The increased milk production is commonly thought to occur from eradicating infections within the mammary gland and preventing damage to the secretory cells. It is well-reported that cows with lower somatic cell counts produce more milk (Hand et al., Reference Hand, Godkin and Kelton2012). The current results are similar to those of (Rasool et al., Reference Rasool, Farooq, Ahmad, Avais and Ambreen2022), who observed a significant rise in milk output in dairy cows with subclinical mastitis following the administration of essential oils (lavender and eucalyptus oil) as a post-milking teat dip. The findings are consistent with those of Shailja and Singh (Reference Shailja and Singh2002), who observed that animals that underwent teat dipping after milking produced a higher amount of milk overall. Similarly, Singh et al. (Reference Singh, Sharma and Singh2018) showed that the post-milking teat dip significantly impacted the teat dipping group's milk production.

There were no significant changes found between control and treatment groups for milk solid not fat, protein and total solids contents as well as freezing point. However, milk fat content and lactose content were significantly lower in GC than in the treatment groups, but only at 45 and 90 DIM (online Supplementary Table S1). These findings are consistent with those of Singh et al. (Reference Singh, Sharma and Singh2018) and Ali et al. (Reference Ali, Patbandha, Odedra, Singh, Garg, Murtuza, Maurya, Agravat and Sarma2022), who reported non-significant changes in milk SNF and total protein contents in treatment groups that underwent post-milking teat dipping as compared to control groups. Lactose is the major osmolar in milk and so at the point of secretion its content is relatively stable. Mastitic infection causes the mammary tight junctions to become leaky, allowing lactose to flow down its concentration gradient out of milk and into blood, which explains the drop in lactose concentration seen in milk. These same leaky tight junctions also explain the changes in potassium (decreased) and sodium (increased) concentrations in milk, which in turn account for the altered EC value (Alhussien and Dang, Reference Alhussien and Dang2018). Our results somewhat coincide with those of Shailja and Singh (Reference Shailja and Singh2002), who found a minor increase in the percentage of milk fat and lactose in the treatment group compared to the control group, although bnin their case the difference was not statistically significant. Comparing the essential oil-based group to the conventional iodine and lactic acid-based post-milking teat dipping solution, the results were equivalent and exhibited no significant differences in any of the above-mentioned parameters.

Teat condition score is a valuable tool for monitoring teat health. It helps identify issues with milking machine settings, assess teat dip concentration and better awareness of the cow's environment, all at little cost. TEC and TSC values did not differ significantly between groups at 0 and 15 DIM (ie, including after 15 d of post-milking teat dipping). However, after 45 and 90 d, both TEC and TSC values were significantly higher in GC compared to the treatment groups (Fig. 1). Skin conditioners such as essential oils are added to teat dips to prevent water loss and protect the skin from chemicals. However, this emollient action keeps teats wet, but it may be detrimental in cold winter conditions. To prevent this, teats should be blotted dry before being let out in chilly environments. Corresponding results were reported by Morrill et al. (Reference Morrill, Scillieri Smith, Dann, Gauthier and Ballard2019), where the 1% iodine-based postmilking teat dip had a better teat end score than the control group.

Mastitis infections are primarily caused by bacteria entering the mammary gland through the teat canal. The teat-end protects the mammary gland, making it the first line of defence against IMI. The extent of bacterial contamination and penetration into the teat canal may be influenced by the kind and extent of teat tissue injury. We examined the relationship between teat-end TEC and TSC values and found a significant although not especially high correlation (r = 0.413). Additionally, there was a correlation (P < 0.01) between SCC and teat end condition (r = 0.374), but not between teat skin condition and SCC (r = −0.023). We did observe that mastitis was more prevalent in teat-ends with significant wear down, which were raw and ulcerated, affecting the TEC scores. However, whilst we found a significant correlation between TEC and clinical mastitis (r = 0.447, P < 0.01), the same was not true for TSC (r = 0.014). The link between a range of degrees of teat-end hyperkeratosis and the rate of udder infection is less clear. The level of teat-end thickness was higher in the clinical mastitis quarter than in the healthy quarter. When the cow's teat ends are abnormal, the probability of new mastitis infection increases considerably. This highlights the importance of healthy teat-end in commercial set-up. It is very helpful to evaluate teat-end callosity which may help in identifying and resolving issues and problems related to milking management. The key characteristics of normal teat ends are smooth and soft, while abnormal ends are chapped, cracked, red or have open lesions.

In conclusion, a post-milking essential oil based teat dip is equally effective as established iodine and lactic acid products in management strategies to reduce new intramammary infection rate in dairy cows. All three post-milking teat dipping solutions can be considered effective and safe, as the product was not irritating to the teat and did not negatively affect teat condition when compared to the control group.