The Delhi High Court, in its decision dated May 7, 2025, in the case of Crystal Crop Protection Limited vs Safex Chemicals India Limited & Ors [CS(COMM) 196/2024, 2025:DHC:3382], clarified the approach for determining patent infringement under the ‘Doctrine of Equivalents’. The Court emphasized that the first step in this assessment is to ascertain the ‘inventive concept’ of the suit patent. The Court also underscored that complete specification of the suit patent plays an important role in determining its ‘essential features’ and ‘inventive concept’. The Court also reaffirmed the principle of estoppel, holding that if a patentee has themselves identified in the Complete Specification certain features that are essential to the patent, they are barred from later arguing otherwise so as to establish infringement.

In the instant patent infringement suit, the Plaintiff had filed an interlocutory application seeking the relief of interim injunction restraining the Defendant from infringing the Plaintiff’s patent number 417213, entitled “Weedicidal Formulation and Method of Manufacture thereof”, marketed as ‘ACM-9’. The suit patent disclosed a novel herbicidal composition comprising ‘Clodinafop 9% and Metribuzin 20%’ (in a ratio of 1:2.2), in combination with a surfactant, a dyeing agent or pigment, and a safener. The patent was asserted to offer enhanced weed control, reduced phytotoxicity, and prolonged efficacy, particularly in wheat crops.

The Plaintiff alleged that the Defendant was infringing the suit patent by manufacturing and selling products under the names ‘RACER’, ‘Trophy’, and ‘Jodi No.1’, which contained the same herbicidal composition covered in the suit patent.

The Defendant disputed the claim of infringement, asserting that their products did not fall within the scope of the patent claims because their formulation did not contain any dyeing agent or pigment, which the Plaintiff had explicitly described in the complete specification to be an essential component of the suit patent. They argued that for infringement to be established, every single element of the patent claim must be present in the impugned product, and the absence of a dyeing agent, in the instant case, precluded any finding of infringement.

The Defendant also challenged the validity of the suit patent on account of lack of novelty, lack of inventive step, and non-patentability under Section 3(d).

However, the Plaintiff attempted to clarify that the suit patent disclosed the presence of dyeing agent as an optional component (an adjuvant or excipient), not an essential component of the invention. They argued that the essential feature of their patent was the composition of two herbicides (Clodinafop Propargyl 9% and Metribuzin 20%) in a specific ratio of 1:2.2, rather than any other aspect such as the presence of the dyeing agent. According to the Plaintiff, the specification and abstract of the suit patent did not present ‘dyeing agent’ as an essential feature, but rather as another embodiment of the invention. Dye or pigment, they argued, was only described to be useful in assisting illiterate farmers in visually assessing the coverage of the herbicide on crops, not as a contributing factor to the efficacy of the formulation or as addressing any technical problems identified in the prior art. They also pointed out that the Defendant themselves referred to ‘dyeing agent’ as an ‘adjuvant’ and excipient/additive in the affidavits submitted by them. They invoked the Pith and Marrow Test asserting that despite minor variation, the Defendant’s products embodied the substance or the essential features of the suit patent.

The Defendant maintained that ‘dyeing agent’ was an essential feature of the suit patent in view of the disclosure in the complete specification, the Plaintiff’s submissions before the Controller (where the presence of the dyeing agent was used to distinguish the suit patent from prior art), and the prosecution history of the patent.

The Court made the following observations:

• When literal infringement cannot be established, the Court ought to apply the Doctrine of Equivalents.
• To assess infringement under the Doctrine of Equivalents, the Court ought to first ascertain the ‘inventive concept’ underlying the suit patent; whether the impugned product achieves the same result in substantially the same way as disclosed in the ‘inventive concept’ of the suit patent.
• Complete specification plays an important role in determining the ‘essential features’ and the ‘inventive concept’ of a patent.
• The inventive concept of an invention must be identified by analysing the claims in conjunction with the complete specification, while also considering the prosecution history.
• Examining prosecution history serves two functions: (a) it plays a significant role in claim construction, (b) it establishes prosecution history estoppel.
• The essentiality of a particular feature in the patent is confirmed if it solves a particular problem identified in the prior art.
• If a patentee has themselves identified in the complete specification certain features that are essential to the patent, they are estopped from later arguing that those features are not essential so as to establish infringement.
• Although ‘adjuvants’ are not principal components of a formulation per se, they play a key role in augmenting the effect of the principal ingredients.

The central question before the Court, in the instant case, thus, was whether ‘dyeing agent or pigment’ was an essential element of the suit patent. In addressing this question, the Court examined the disclosure in the complete specification of the suit patent, particularly in:

•  The “Background of the Invention”, where the Court found that the invention aimed to solve the issue of difficulty in visually assessing the effectiveness of applied herbicides on weeds.
• The “Objects of the Invention”, where the Court found that one of the primary objectives of the invention was to enable farmers to visually assess the herbicide’s impact on weeds.
• The “Detailed Description of the Invention”, where the Court found that dyeing property was described as one of the unique concepts of the invention.

Building upon this, the Court proceeded to analyse the claims of the suit patent, noting that ‘dyeing agent’ was expressly included in two independent claims (a product claim and a method claim). Referring the Manual of Patent Office Practice and Procedure, the Court reiterated that an independent claim includes all features essential to define the invention.

Based on the findings, the Court observed that the presence of ‘dyeing agent’ in the claims was not merely incidental but served a distinct functional purpose of enabling visual identification of treated areas. The Court noted that this feature directly addressed the problem of assessing herbicide application effectiveness, as highlighted in the specification. The Court emphasized that once a feature is explicitly claimed, and in the absence of any clear disclaimer, it must be regarded as an integral part of the inventive concept.

Examination of prosecution history revealed that ‘dyeing agent’ was originally a part of the dependent claims but was later, post hearing, incorporated into the independent claims to overcome the Controller’s objections. The Plaintiff, during post-grant opposition proceedings, had also differentiated their invention from prior art by specifically emphasizing the presence of ‘dyeing agent’.

The Court remarked that at no stage during prosecution did the Plaintiff claim that ‘dyeing agent’ was an optional component and this assertion surfaced only during the infringement proceedings. The Court held that the Plaintiff could not be permitted to ‘approbate and reprobate’, meaning they could not adopt contradictory positions at different stages. Given the clear position taken during prosecution, the Court concluded that the Plaintiff was bound by its initial representation of ‘dyeing agent’ as an integral part of the inventive concept of the suit patent.

Ultimately, the Court concluded that ‘dyeing agent or pigment’ was an essential feature of the suit patent and held that the Plaintiff failed to establish a prima facie infringement case.