Milk contains lactose which on fermentation produces lactic acid. Since, milk contains proteins, and almost all minerals (except Fe), it may suffice part of the nutritional requirements of the plants creating a slight stress within the plant as well. This will also trigger secondary metabolites which may further aid in crop improvement. Moreover, fermented milk (curd) is easily decomposable by micro-organisms, some even beneficial to plants and will increase their population in the rhizosphere.

Copper used as catalyst will stay in the curd and may act as a co-factor for many  beneficial enzymes. So one can expect more release of enzymes from the plant system resulting in mobilization of unavailable nutrients into plant-available form in the rhizosphere.

It is highly unlikely that this has been assessed but opens up some interesting research areas.

Thank you sir