Reliability of the Kernel Milkline Method for Timing Corn Silage Harvest in Ohio

The position of the kernel milkline (ML) has been recommended as a reliable guide for determining the optimal time to harvest corn (Zea mays L.) for silage. This method has not been tested adequately in the central and eastern Corn Belt, and the effect of planting date on variation in whole plant dry matter (DM) content at different ML stages is unknown. The objective of this research was to determine the variation in whole plant DM content at different kernel ML stages of corn grown in Ohio. Three hybrids representing early, mid, and full season maturities for central Ohio were planted on or about 1 May, 21 May, and 10 June 1992 and 1993. Kernel ML stage and whole plant DM content were determined every 3 to 7 d beginning at full dent (FD) stage until ML disappearance. Kernel ML stage and whole plant DM content were positively correlated (r = 0.89). On average, the kernel ML would have been an excellent indicator of when to harvest corn for silage; however, we found considerable variation in plant DM content within a given ML stage for individual sampling dates from all hybrids and environments. Variation in DM content was present regardless of year, planting date, or hybrid. Although corn was more likely to be at an ensilable DM content at certain stages than others, the variation in plant DM content within a stage was sufficient to raise concern over the reliability of the kernel ML method for determining when to harvest a particular field for silage in Ohio. Gravimetric determinations of plant DM content should be used to confirm when corn is at the optimal DM content for ensiling. Corn silage growers in Ohio should begin making gravimetric determinations of plant DM content at FD for conventional tower or bunker silos, and 1/4 ML for sealed (oxygen-limited) tower silos. Research QuestionDetermining the proper time to harvest corn for silage is critical because whole plant dry matter (DM) content varies with maturity and influences harvesting and fermentation losses. Observing the position of the kernel milkline (ML) has been recommended widely as a reliable visual guide for determining the optimal time to harvest corn for silage. This method has not been tested adequately in the central and eastern Corn Belt. Although much of the corn grown for silage is planted later than for grain, the effect of planting date on the relationship between kernel ML stage and plant DM content in corn is unknown. Planting date may influence the variation in whole plant DM content at different kernel ML stages. The goal of this study was to determine if kernel ML stage is a reliable indicator of when to harvest corn for silage in Ohio. Literature SummaryCorn silage preserved between 30 and 40% DM content generally provides good fermentation and animal performance; however, different storage structures require different target DM concentrations for good fermentation. Generally accepted recommendations for com silage DM are: 32 to 40% for conventional tower silos, 40 to 45% for sealed (oxygen-limited) tower silos, and 30 to 35% for bunker silos. Researchers in Minnesota, Wisconsin, Idaho, and California have recommended that corn be harvested between 112 ML stage (ML positioned halfway between the tip and base of the kernel) and 3/4 ML stage to black layer formation to achieve DM concentrations between 30 and 40%. In Pennsylvania, however, researchers reported that DM content of corn plants was higher than 40% after the 1/2 ML stage. They recommended corn silage be harvested between full dent (FD) and 1/2 ML stages in the northeastern USA, adding that adjustments might be needed for different hybrids and local growing conditions. Study DescriptionField experiments were conducted in 1992 and 1993 using three hybrids planted on or about 1 May, 21 May, and 10 June. The hybrids ranged from 106 to 115 d comparative relative maturity, which encompass the range in maturity commonly found in hybrids grown commercially throughout Ohio. Kernel ML position and whole plant DM content were determined every 3 to 7 d beginning at FD stage and continued until the ML was no longer present in the kernel. On the average, 1992 was cooler than 1993, and both years were drier than the long-term average. Kernel ML stages sampled in the study: FD = all kernels dented with solid endosperm just visible near the top of the kernel 1/4 ML = milkline positioned 114 of the way from the tip to the base of the kernel 1/2 ML = milkline positioned halfway between the tip and base of the kernel 3/4 ML = milkline positioned 314 of the way from the tip to the base of the kernel No ML = milkline no longer present in the kernel. Applied QuestionsIs kernel ML stage a reliable indicator of when to harvest corn for silage? There was a high correlation between kernel ML stage and whole plant DM content, and corn was more likely to be at an ensilable DM content at certain stages than others. For example, about 70% of the samples taken at 114 ML fell within the 32 to 40% DM content range recommended for conventional tower silos (Fig. 1). Nevertheless, harvesting corn at a given ML stage in Ohio did not guarantee it was always at an ensilable DM content. We found considerable variation in plant DM content within each ML stage for individual sampling dates from all hybrids and environments (Table 1, Fig. 1). Variation in plant DM content within a ML stage was present regardless of year, planting date, or hybrid. This variation was sufficient to raise our concern over the reliability of the kernel ML method for determining the proper time to harvest a particular field for silage in Ohio. Gravimetric determinations of plant DM content (e.g., using a microwave or commercial forage moisture tester) should be used to confirm when corn is at the optimal DM content for ensiling. The kernel ML can be used as a guide of when growers should begin to make these gravimetric determinations. At what corn kernel stages should growers begin taking forage DM tests? Based on our data, corn silage growers in Ohio should begin making gravimetric determinations of plant DM content at FD for conventional tower or bunker silos, and 1/4 ML for sealed (oxygen-limited) tower silos. These stages are slightly earlier than what we most often expect to be the ideal stage for ensiling corn for each type of structure, but it is necessary to begin sampling early because of the possible variation in plant DM content within a particular ML stage. Our data agrees with a previous Pennsylvania study, indicating that the recommended ML stages for corn silage harvest in northern states (e.g., Minnesota and Wisconsin) are not appropriate for Ohio. At any given ML stage, corn silage in Ohio is likely to be drier than that in Minnesota and Wisconsin. Table 1Plant DM content of corn at different kernel ML stages, over three hybrids and six environments in Ohio. StageMeanRangeFull dent30.626.0–41.21/4 milkline35.129.9–42.41/2 milkline38.935.1–44.43/4 milkline43.938.3–48.5No milkline48.642.4–55.6 Fig. 1Whole plant DM content of corn at different kernel ML stages. Each data point represents the mean of three replicates on one sampling date of a specitic year-planting dste-bybrid combination.