Source: University of Minnesota Extension

Ideally, wheat, barley and oats should be seeded at 1 1/2-2 inches of depth. The idea is that the seed should be placed deep enough to have access to adequate moisture yet shallow enough to emerge as quickly as possible. 

Seeds too close to the surface absorb moisture but are at risk of dying because roots cannot reach moisture quickly enough to sustain the germination and seedling growth. Deeper seeding can reduce stand density and plant vigor because the inability of the coleoptile to reach the surface. 

Given the extremely dry conditions, the question arises how deep we dare to drill wheat, barley and oats down to place the seed into moisture. There are several factors to consider, including the crop itself, the variety in question and the soil type into which the seed will be placed. On average, oats is the most tolerant to be being placed deeper than the optimum 1.5-2 inches, whilst barley is the least tolerant.

Varieties of each of the three species differ genetically in the maximum length of their sub-crown internode and their coleoptile. The sub-crown internode moves the crown from the seed towards the surface and whether a sub-crown internode develops is a function of seeding depth.  

Seeding deeper than 1.5 inches generally will result in the development of a sub-crown internode. The maximum length of the sub-crown differs between cultivars and although not a lot of data is available, you can assume that shorter statured varieties have shorter sub-crown internodes. 

Oats is different than either wheat or barley as the internode between the scutellum and coleoptile is also able to elongate, thereby allowing the oats to be the most tolerant to seeding deep (up to approximately 4 inches).

The maximum coleoptiles lengths differ between varieties within each of the species. The average plant height of varieties as reported in the variety trials correlates reasonably well with the length of the coleoptile and can be used guidance to assess the risk of planting too deep. Table 1 summarizes the maximum coleoptile length measured in a growth chamber experiment using germination paper of a number of HRSW and barley cultivars.

Finally, finer textured soils create more resistance for emerging seedling than coarser textured soils, even in the absence of a crust. Therefor you have less leeway to place the seed deeper and into moisture in finer textured soils. The University of Nebraska suggests the difference being about 1/2 inch.

 Bottom line — barley should probably not be seeded much deeper than 2 inches, while many of the semi-dwarf wheat varieties probably should be seeded much deeper than 2 1/2 inches. As said before, oats can probably be seeded as deep as 3 inches without jeopardizing the initial stand.