Increased seed cost of glyphosate tolerant (GT) compared to non‐GT alfalfa (Medicago sativa L.) raises questions about reducing seeding rates. The objective of this study was to determine if alfalfa stand density, yield, and forage quality are compromised in the later years of the stand when reduced seeding rates, in combination with glyphosate application, were used during establishment. At seven locations in the United States, GT alfalfa was seeded into conventionally tilled seedbeds at 6.7, 11.2, 15.7, and 20.2 kg ha−1 pure live seed (PLS) in the spring of 2006. Stand density, yield, and forage quality were determined for each seeding rate under three herbicide treatments: (i) glyphosate [N‐(phosphonomethyl)glycine], (ii) non‐glyphosate herbicide, and (iii) no herbicide. For the third through fifth years of the alfalfa stand, there was no herbicide treatment × seeding rate interaction. Higher seeding rates resulted in higher plant densities. The 11.2 and 15.7 kg ha−1 seeding rates had greater alfalfa and total forage (alfalfa + weeds) yields than the 6.7 kg ha−1 seeding rate. Herbicide treatments had no effect on alfalfa plant density but produced cumulative (over the duration of the study) alfalfa yield in the following order: glyphosate > non‐glyphosate > no herbicide. Cumulative total forage yield was less when no herbicide was used compared to using a herbicide. While forage fiber content was generally unaffected by seeding rate or herbicide treatments, using glyphosate to control weeds during alfalfa establishment increased forage crude protein content of older alfalfa stands compared to no weed control.

Higher costs associated with glyphosate tolerant (GT) compared to nonGT alfalfa ( Medicago sativa L.) seed stimulates questions about reduced seeding rates in combination with the GT technology. Our objective was to determine if glyphosate herbicide in combination with GT alfalfa could improve the persistence, productivity, or forage quality when seeding alfalfa at a reduced rate. Glyphosate‐tolerant alfalfa was seeded at seven locations into conventionally tilled seedbeds at rates of 6.7, 11.2, 15.7, and 20.2 kg ha −1 pure live seed (PLS) in the spring of 2006. Stand density, botanical composition, yield, and forage quality were determined for each seeding rate under three herbicide treatments: (i) glyphosate [N‐(phosphonomethyl)glycine], (ii) a nonglyphosate herbicide, and (iii) no herbicide. Level of weed infestation was different among locations, but there were no weed infestation × seeding rate or herbicide × seeding rate interaction. Lower seeding rates had lower plant mortality than higher seeding rates. Seeding rate had no affect on forage quality or weed content at any harvest. At only the first harvest in the seeding year did the 6.7 kg ha −1 seeding rate produce less alfalfa forage (about 250 kg ha −1 ) than other seeding rates. In both the seeding year and year after seeding, using herbicides resulted in less weed and greater alfalfa yield than when no herbicide was used. Regardless of weed control treatment, seeding rates of GT alfalfa greater than 6.7 kg ha −1 did not improve weed control, alfalfa yield, total herbage (alfalfa + weeds) yield, or forage quality.

The incidence of alfalfa (Medicago sativa L.) pasture bloat appears to increase with growth at the low temperatures that prevail in the high‐altitude valleys of the Intermountain West. This study determined whether plant characteristics associated with bloat are more prevalent in the leaves of alfalfa plants grown at suboptimal (15/5°C) day/night temperatures than in plants grown at optimal (25/15°C) temperatures. We hypothesized that the larger leaves characteristic of alfalfa grown at low temperatures would contain more soluble protein and chlorophyll than leaves of plants of the same cultivars grown at higher temperatures, even though cell number per leaf remains constant. We also expected that the larger cells typical of alfalfa grown at low temperatures would rupture more easily than cells of alfalfa grown at warmer temperatures. Fresh leaves were ground with a mortar and pestle, or shaken in buffer with glass beads to simulate mechanical damage such as would occur with rumination. Leaves from growth chamber‐grown Ranger and Fortress alfalfa contained more chlorophyll and more than twice the soluble protein when grown at low than at optimal temperatures. Contrary to our expectations, susceptibility of cells and tissue to damage decreased by approximately 15% with growth at low temperatures. Soluble plant protein and chlorophyll have been implicated in alfalfa bloat, and the increased content of cellular components related to bloat with low temperature growth could increase the rate of their release into the rumen, resulting in bloat.

Exposure of alfalfa (Medicago sativa L.) to frost reportedly increases the frequency of bloat in ruminants grazing the forage. Our objective was to investigate changes in leaves of the plant that occur with freezing that could be related to ruminant bloat. A high occurrence of bloat is associated with elevation in the ratio of potassium (K+) to sodium (Na+) in the rumen, which is thought to cause flocculation of chloroplast membrane particles, producing a stable foam that results in bloat. In several plant species, exposure to frost temporarily inhibits ATPase activity, resulting in the movement of water and K+ from the protoplast of plant cells into the apoplast. Our hypothesis was that an overnight frost causes movement of K+ into the apoplast of alfalfa leaves, and leaching of this extracellular K+ from the foliage would increase the ratio of K+ to Na+ in rumen fluid of livestock grazing the alfalfa. To test this hypothesis, we slowly froze detached alfalfa leaves to −2°C, and nucleated extracellular ice formation. We then thawed the leaves and leached them in distilled, deionized water. The mean increase of 1.061 g K+ kg−1 t leaf flesh weight leached from frozen‐thawed alfalfa was sufficient to increase the occurrence of bloat according to evidence from other studies. Compared with control leaves, calcium and magnesium were also elevated by 1.821 and 0.425 g kg−1 leaf fresh weight, respectively, in leachate from frozen‐thawed alfalfa, but relatively little Na+ was found in this leachate. A similar increase in leachate ion concentration was found when the study was repeated.

Revised September 1981. Facts and recommendations in this publication may no longer be valid. Please look for up-to-date information in the OSU Extension Catalog: http://extension.oregonstate.edu/catalog