Overall, our findings indicate that application of the BSF frass fertiliser HexaFrass (HF) significantly increased shoot height and weight of hemp seedlings compared with that achieved in no-fertiliser control treatments. The occurrences of these positive effects were generally consistent, in that they were observed across three monoecious varieties of hemp and in male and female plants of three dioecious varieties. These results align with previous studies where soil amendments created from silkworm (Bombyx mori L.) frass also increased the dimensions, growth rate, and yield of hemp shoots [38,39]. Additionally, HF produced similar effects on hemp growth as those obtained with two commercially available organic fertilisers (chicken manure, Miracle-gro). Several previous studies also reported that the application of IFFs produces equivalent effects to standard fertilisers, both in glasshouse trials investigating herbs, vegetables, grasses, and cereals and in field trials using maize [16,17,19,20,21].

The positive effects of HexaFrass on plant performance were observed at relatively low application rates, and the application of additional HF did not result in further hemp shoot growth. Several previous papers have also reported polynomial or asymptotic relationships between hemp growth and fertiliser application rates [40,41,42,43,44]. Yang et al. [45] reported that an increase in applied N generally promoted hemp growth, but excessive supply could reduce nitrogen use efficiency and/ or plants could be weakened by oxidative stress. Similarly, non-linear relationships between plant growth and IFF application rate have been reported for several plants, for example, tomatoes [46], forage plants [17], herbs [16], grasses [19], and cereals [21].

For female plants, shoot Fwt, Dwt, and Ht levelled off at approximately 4 g HF pot, and this application rate was adopted in all subsequent trials. For male plants, there was some indication that at the highest application rates used (18–20 g HexaFrass per pot), plant growth started to be stressed, although we did not observe the pronounced inhibition of shoot growth or plant death as seen in some other studies [21]. Currently, we are unsure of the mechanism behind the lack of additional shoot growth in hemp plants treated with high IFF application rates, but factors such as plant density, root zone compaction, the build-up of bioactive chitin, surplus nutrients, or the production of excess ammonium via mineralisation have all been mooted and require additional research [47,48,49].

It is also possible that applying the total dose of IFF to young hemp seedlings in a single event meant these plants could not utilise all the available nutrients before leaching or breakdown of the fertiliser had occurred. Hemp may respond positively to boosts in nutrients at the flowering, budding, or seed formation stages, and future research should examine the application of IFFs at different life stages and/or compare single versus split IFF application regimes. From the above discussion, if IFFs are to be used in commercial hemp production systems, it would be necessary to ascertain the optimal application rate, and fertiliser regime, in terms of both plant performance and economic considerations.

With a view to using agricultural hemp as livestock forage or as a component of mixed rations, application of HexaFrass increased both the biomass and the nutrient density of foliage and stems. Overall, the hemp seedlings treated with HexaFrass had higher concentrations of phosphorus, potassium, and magnesium in the above-ground tissues, although calcium and nitrogen were not similarly increased. Previously, the application of IFF has been shown to have mixed effects on the nutrient properties of plant foliage. For example, application of IFF has resulted in increased N, P, and K in barley straw [21]; increased N but not P and K in spinach [50]; and increased K in sunflower leaves but with negligible effects on N and P [51]. Although not a primary aim of this investigation, future work would benefit from a more detailed analysis of IFF-treated hemp material in terms of its value as livestock fodder, for example, by quantifying components such as protein, carbohydrates, energy content, and digestible fibre content [52,53].

Although the occurrence of positive effects on shoot growth occurred consistently in all trials, across all cultivars, and for both male and female plants, the magnitude of these responses demonstrated both systematic and seemingly random heterogeneity. Previously, differences in the size of growth effects achieved by IFFs have been attributed to factors such as the plant taxa tested, the insect species producing the frass, and the composition of the rearing diet [16,20,24,53]. In our trials, systematic differences in responses to HF were clearly apparent between male and female dioecious hemp plants, in that female plants within each cultivar accumulated greater shoot Dwt than the male counterparts when HF was applied. The male hemp plants were significantly taller than the female plants, and it was generally the case that the female hemp plants were shorter and bulkier than the male plants [54]. The female plants, although shorter, accumulated disproportionate shoot Dwt than the male plants in response to HF application, accentuating the differences in dry weight per unit length of shoot. The phenotyping, and separation of, male and female hemp plants treated or not treated with IFF is of interest, and could be performed using morphological parameters, plant chemistry, or phenological events such as germination, flowering, and seed set.

Even when grown under relatively consistent conditions, using the same cultivars, and using the same growing media, hemp plants can exhibit high levels of phenotypic heterogeneity [55,56]. Previously, Wang et al. [22] reported differences between trial years in the response of rice to BSF fertiliser, and Rodgers et al. [19] described variation among their trials when HF was applied to perennial rye grass. Within a wider framework of reproducibility of scientific results [57], and because of inherent inconsistencies in the quality of organically-derived fertiliser products, repeated, standardized testing is highly desirable to provide information on the generality of effects, the consistency of effect sizes, and, therefore, what might be considered a ‘typical’ response [58,59,60]. The use of standardized effect sizes, such as Cohens d and Hedges g, can be useful tools in this regard, as they allow between-trial heterogeneity to be quantified within the same study (as in our case), in addition to identification of factors contributing to heterogeneity in effect sizes across different studies [37,61].

We concede that our investigation represents a preliminary study, and that the results presented were obtained from plants grown over a short period, in small containers, under benevolent glasshouse conditions. Nevertheless, several authors have indicated that, as commercial IFFs are relatively novel plant-promoting products, it is necessary to gain initial empirical data across a wide range of plant groups so as to identify commonalities and situations where variation occurs [50,62]. The scaling up of experiments to larger, longer-term trials, utilising a range of soil types and environmental conditions is clearly required to provide additional evidence of the value of IFFs for sustainable hemp production. Additionally, several authors have reported benefits of IFFs centred around enhanced stress resistance and/or activation of defences against invertebrate pests or pathogens [9,26,28,63]. As many of the high-value secondary metabolites produced by hemp are associated with plant defences, the effects of IFFs on terpene and cannabinoid profiles would have high biological and economic relevance. Finally, recent papers have investigated the value of hemp waste as a potential feedstock for the production of black soldier flies and mealworms [64,65]. If IFFs can be used to increase hemp biomass on a commercial scale, and the waste from this hemp is then repurposed as a rearing medium to produce more insect protein and frass-based fertilisers, this would epitomise a circular bioeconomy system with respect to sustainable agricultural production.