When you say "farmyard" manure, what animals arte involved?

For example, fresh chicken manure can cause burning of seedlings or root damage.

Excessive nitrogen from any source can sometimes have negative effects on maize. It may cause damage root damage as mentioned above, but it might also promote leaf growth at the expense of seed production. In other words, more foliage, less seed.

Did plants show any sign of damage or stunting or did they look very healthy?

Any kind of fresh manures can cause problems in soils and plants, before you use need to be properly composted.

The definition of Farm-yard-manure FYM should be: matured balanced mixture of ALL animal-droppings with straw (or other bedding-material). Any excessive straw-share or too concentrated dropping will negatively impact the rhizosphere, thus plant-roots. FYM has been ---- beside crop-rotation with leguminous crops and fallow --- the main factors / preconditions of soil-fertility of most human civilization in the past 5 millennia. If FYM reduces crop-yields, the agronomist in charge needs some further training.

Rightly pointed out that this findings as not very common one . There are three scientific reasoning to in thd support of this phenomena. 1.Application of partiality decomposed FYM. 2. Use of fym without taking consideration of OC contain of the particular field. 3. Faulty calculation of nitrogen requirements, without taking proper account of soil fertility status and nutrition requirements of the crop in question.

Todos os resíduos orgânicos, seja estercos, palhas, folhas, restos de culturas agrícolas, devem ser primeiramente passar por um processo de compostagem para mineralizar os nutrientes e estabilizar a matéria orgânica. Dessa forma, o uso de composto orgânico como adubo natural vai estimular o desenvolvimento da cultura do milho (exemplo) e aumentar a produtividade da cultura.

Was it the yield reduction in FYM used plot over control (no FYM used) plot? .

In some case, the yield reduction due to partially decomposed cowdung/high application may cause toxic to plants

Fresh farmyard manure with a large C/N ratio will result in a temporary N depletion caused by microbial growth in response to manure application

The quality and source of the farm-yard manure are important determinants of how much yield be realized. For example, if the manure was stored in the open for a long time and was rained on, it loses the essential nutrients needed for crop growth.

I am agreed with opinion of Dr. Charles, undoubtedly Open heap system leached some of its nutrients, but also helpful in retaining mesophilic temperature during decomposition process and protect microbial community on it.

Because most of the Fungi and bacteria which are helpful in agricultural point of view, grow well on mesophilic temperature and help to improve nutrient exchanges! Reduction in crop yield with application of farmyard manure is probably  due to application of immature organic manure or high quantity of manure.