Sorghum (Sorghum bicolor subsp. bicolor) originated from the African continent, where it continues to be an important crop for grain and forage. Within the Sorghum genus, there are a number of species important in agriculture. This thesis will largely be limited to the types of sorghum commonly used for forage. Forage sorghum is widely grown throughout many climatic regions of the world, primarily for feeding ruminant livestock. Some of the earliest references to members of the sorghum family being used for forage purposes in Australia are from the early 1900’s, when Johnson Grass (Sorghum halepense) was introduced for use as a fodder species. The development and introduction of hybrids in the 1960’s was a major step forward and led to much greater use of forage sorghum in the livestock industries.
Forage sorghum is an excellent forage crop for ruminants but for over 100 years it has been known that it can be poisonous under some circumstances. Due to the significant contribution of forage sorghum to livestock feeding, various research studies were conducted through much of the 1900’s, to develop management strategies to more safely use this forage crop. One of the key strategies that persisted through much of this period was to wait until the plants were at or near flowering before feeding to livestock, which resulted in forage being fed when it had significantly lower nutritional value. Despite the efforts to understand and manage forage sorghum, outbreaks of animal intoxication have continued to occur. The cause of this toxicity is most often attributed to prussic acid or hydrogen cyanide. In cases of animals dying while grazing forage sorghum or eating sorghum hay, livestock owners and veterinarians usually assume the cause is cyanide poisoning. However, as this study highlights, forage sorghum can also accumulate toxic levels of nitrate. Many of the agronomic and environmental factors that increase the risk of cyanide poisoning also lead to elevated nitrate content. As the toxicity symptoms from both these toxins are relatively similar, it is not surprising in cases of intoxication that the simplistic conclusion of cyanide poisoning has been made. Much of the agronomic advice on cyanide poisoning included in current advisory bulletins and even textbooks can be attributed to conclusions made in reports more than 40 years ago. The conclusions and advice have been simply transcribed from publication to publication without question. There is confusion as to the effects of sun curing and ensiling on the levels of these two compounds. Aside from the implications of these compounds for livestock, there has also been a lack of appreciation of their roles and relationship in sorghum plant metabolism. Due to observed discrepancies in the published literature, the improvements in cyanide analytical techniques and the changes in farming and fodder conservation practices, further investigation was warranted. The studies in this thesis were therefore initiated to examine the influence of forage sorghum growth and subsequent processing (hay curing and ensiling) on toxicity and the relationship of the two compounds during drought and drought recovery conditions. Several series of agronomic and fodder conservation trials (hay and silage) were conducted using Australian commercial forage sorghum hybrids. In all, 20 experiments were conducted using a range of hybrids under both greenhouse and commercial field conditions.
The results from these studies have clarified several issues concerning the toxicity of forage sorghum when made into hay or silage. The results clearly show that hydrogen cyanide potential (HCNp) is not reduced as a result of hay curing. There may in fact be an increase in HCNp as a result of metabolic activity in the cut plants during the early stages of sun curing. These findings have implications for farmers cutting crops of forage sorghum for hay. In situations where the HCNp is relatively high but still below the potentially toxic level of 800 μg/g dry matter (DM), metabolic activity after cutting could result in the HCNp increasing to a toxic level.
Ensiling has been shown to reduce HCNp by up to 80% when the ensiling period is at least 12 months. However, contrary to many advisory bulletins, the results from these studies show that ensiling should not be relied upon to reduce the nitrate content of forage.
Another widely held belief is that drought stressed forage sorghum has a toxic HCNp and should not be fed to livestock. It has been shown that plants which have suffered prolonged severe drought can actually have a non-toxic HCNp, allowing such crops to be used during those times of severe drought and feed shortage. Caution is still required, particularly as nitrate levels may be elevated, but it means drought affected forage sorghum may in fact be safe to use, providing fodder in times of severe shortage.
One of the other aims of this work was to investigate the possibility of an association between dhurrin and nitrate in forage sorghum. Both of these nitrogen containing compounds can accumulate in forage sorghum under specific environmental conditions, such as drought or any condition which slows plant growth. Dhurrin is generally regarded as a chemical defence compound in sorghum and nitrate as a key component of nitrogen metabolism. When classified in this manner, plants with limited nitrogen (due to soil nitrate content or drought) would be assumed to face the dilemma of allocating nitrogen to defence (dhurrin) or to nitrate for growth. However, as nitrate can also be toxic to herbivores, the potential secondary role of nitrate as a chemical defence compound in sorghum cannot be ignored. The levels of dhurrin and nitrate were studied in drought affected plants and in the days following rain or irrigation. The results showed that within 1 or 2 days of increased soil moisture, the HCNp decreased and nitrate increased. The evidence supports dhurrin being degraded to supply readily useable nitrogen for critical needs, while any nitrate absorbed from the soil temporarily accumulates to fill the gap as a chemical defence. The potential for dhurrin and nitrate to each have dual roles in chemical defence and nitrogen metabolism is discussed. The nitrogen metabolism situation is dynamic and the balance of where to access nitrogen and what to use for defence, can change each day, as uptake and reduction of nitrate resumes and new growth begins.
The present studies have highlighted the complexities of investigations relating to forage sorghum toxicity and have demonstrated the diversity of data (both laboratory and field) required when making an assessment. A more complete understanding of HCNp and nitrate metabolism by the plant both before and after harvest and the application of rigorous analytical protocols will reduce the risk of exposing livestock to toxicity when grazing forage sorghum or consuming conserved forage sorghum.