Fall/winter harvest/grazing management of sorghum forages

This is a write up by our Forage Crop Management Specialist, Leonard Lauriault, on harvest management of sorghum forages when they are frosted.  FYI, our State Extension Forage Specialist, Mark Marsalis, published an excellent resource entitled “Sorghum Forage Production in New Mexico” (Guide A-332, http://aces.nmsu.edu/pubs/_a/A332/).

Generally, there are two concerns with harvesting sorghum forages as hay or silage or by grazing when temperatures cool in the fall and especially when frost occurs. Those are nitrate accumulation in the stems and Prussic acid production in leaves. The causes are the same so they can both occur at the same time.

Because of the within-plant location of the two concerns, and the fate, or lack thereof of the toxins, different management strategies can be used.  Nitrates are more of a problem for hay, unless animals are forced to consume the stems during grazing. Prussic acid is more of a concern when grazing because leaves will be consumed first.

Nitrates never go away in hay, although ensiling will reduce nitrate levels as microbes utilize the nitrates. Prussic acid will naturally dissipate in 5 to 7 days  and actually is generated by cutting for hay or silage at any time during the year. Dissipation is usually complete by the time hay is cured or after 7 stress-free days have passed for the standing crop. Nitrates accumulated during stress (cold temperatures or frost) also can be assimilated by the rest of the plant after about a week of stress-free growth.

The problems with sorghum forages in the fall are temperature fluctuations and frequent frosts.

Consequently, the recommendation for grazing sorghum forages in the fall is to remove animals when frost is first anticipated and not return them until at least a week after all plants are completely dead. Then, never force them to graze the stems because the nitrates are still there. Alternatively, as soon as frost is anticipated, the sorghum forages can be harvested. This would prevent nitrate accumulation due to frost and any Prussic acid that is produced due to cutting should dissipate before baling.

The forage nitrate screening kits I provide each April (which are still good until next April 1^st, not fooling) can be used to evaluate the presence of nitrates in sorghum stems and how high up the stems the nitrates have accumulated. If you have a kit, you can have producers collect stems cut at ground level in several places throughout the stand (the more sampling locations, the better). Have them identify the collection location for each stem. Split the stems and apply a drop of the nitrate screening solution about every two inches up the stem. If nitrates are present, the drop will immediately turn a deep blue. The point at which there is no blue will indicate a possible cutting height to leave most of the nitrates in the field. This could also leave considerable hay yield in the field, but I suspect most producers will consider their animals of greater value.Also, any nitrates left in the field may be available to a subsequent crop as a fertilizer savings. Identifying sample collection location may give an idea of areas where nitrates are higher or lower in the field due to soil or microclimatic factors. Hay from high nitrate areas could be sold separately to protect the value of any low nitrate hay.

Because nitrates never go away in hay, in every case in New Mexico, no matter how well the sorghum forage crop was managed or how optimum the growing conditions were, every cutting of sorghum hay should be tested for nitrates. The forage nitrate screening kits I provide only indicate the presence of nitrates and not the amount, and amount counts. Management options to utilize high nitrate hay have been devised based on the forage nitrate level. Forage quality analysis also should be done as a marketing tool. Proper hay sampling technique is critical to estimate the nitrate content or nutritive value of any forage with any degree of confidence (see Circular 641, http://aces.nmsu.edu/pubs/_circulars/CR-641.pdf, also by Mark Marsalis).

By the way, sorghums also produce nitrates and prussic acid until new plants are about 18-20 inches tall. Consequently, after planting or harvesting, they should not be grazed until they are about 2 feet tall. Cut for hay at 40 inches to maintain fine stems for higher quality and more rapid curing. Silage should be harvested at soft dough stage and may need to be swathed to allow wilting to 65% moisture before chopping.

An alternative to sorghum for forage is pearl millet (see Guide A-417; http://aces.nmsu.edu/pubs/_a/A417/, by guess who). Management practices are similar, except that pearl millet does not produce Prussic acid, and although it can accumulate nitrates, pearl millet is not generally as likely to do that as the sorghum forages (even alfalfa can accumulate nitrates). Consequently, pearl millet does not present the same concerns during the fall that the sorghums do. Of note is that pearl millet does not withstand intensive grazing (lower stubble height) as well as the sorghum forages for regrowth, but if you’re grazing it out in the fall, who cares about regrowth.

Eric Scholljegerdes is conducting the second year of a grazing trial at Tucumcari that may shed further light on the difference in animal performance when using pearl millet or sorghum forages for late summer and fall pasture (thanks, Eric and graduate student Leah Schmitz). We’ll keep you posted as results are substantiated.

Mr. Leonard Lauriault, Certified Forage and Grassland Professional

College Professor - Superintendent  and Forage Crop Management Scientist
New Mexico State University College of Agricultural, Consumer and Environmental Sciences: http://aces.nmsu.edu/
Plant and Environmental Sciences Department: http://aces.nmsu.edu/pes
Agricultural Science Center at Tucumcari: http://tucumcarisc.nmsu.edu/
6502 Quay Rd AM.5
Tucumcari, NM 88401
Phone: 575-461-1620 x 103
FAX: 575-461-1631

 Perseverance is the pathway to perfection (Philippians 3:12-16).