Can grazing selectivity increase fatty acid and nutritive content of annual forages grown for dairy?

This research was conducted at the Choiniere Family Farm in Highgate Center, Vermont.  Guy and Matt Choiniere milk about 90 cows on their farm, certified organic in 2002.  In 2014, the Choinieres switched to producing “grassmilk” for Organic Valley Co-op.   The Choinieres incorporate various annual forages as a supplement to their permanent cool season pastures.   This project involved three separate studies described below that were conducted on their farm.  With these studies, seeding choice and rate as well as soil amendments, manure and grazing practices were all in accordance with the Choiniere’s current practices.   Briefly; dairy cattle were appropriately fenced to strip graze the annual forages – winter rye in early spring, and warm season annuals in mid-summer – for approximately one to two hours prior to their P.M. milking.  The annual forages were strip grazed from one end of the field to the other – therefore daily strip grazing progressed from forage in an early vegetative stage at day 1 to a later elongation or early reproductive stage by the time the cows reach the other end of the field. 

Study 1. Winter Rye Quality and Maturity - In early fall of 2015, a two-acre strip (primarily silt loam) was seeded to winter rye to be used for supplemental grazing in early spring 2016.  In spring of 2016, sampling began the same day but just prior to the first day of grazing when the stand was in early elongation stage approximately 16 inches in height.  The strip was also sampled on Day 5 (boot stage) and 9 (head emergence) within each area to the grazed, respectively.  The time interval was quite short due to the rapid maturity of the rye.  During each sampling date, five replicate samples were randomly collected within the strip area using a 2.5 square foot quadrat and handheld electric clippers.  Rye forage was cut to a stubble height of two inches.   As soon as possible after sampling (within 30 minutes), each sample was divided into leaf blade and pseudostem fractions, separated just above the ligule.  To degrade any enzymatic activity that might reduce fatty acid content or water soluble carbohydrates, a 150 gram subsample of each fraction sample was heated in a microwave for one to two minutes soon after sampling.  They were then transported to the UVM Horticultural Research Farm where they were dried at 60 C in a forced hot air dryer for 48 hours. 

After complete drying, samples were then weighed to establish dry matter weights and yield of both fractions, ground with a Wiley mill (Arthur H. Thomas, Philadelphia, PA) to pass through a 2mm screen, and then a cyclone forage mill (UDY Corporation, Fort Collins, CO) to pass through a 1mm screen, and analyzed for forage quality using Near Infrared Reflectance Spectroscopy (NIRS) according to the procedure developed by the NIRS Consortium (NIRSC, Hillsboro, WI) program of the National Forage Testing Association (NFTA, Avoca, NE).  Fatty acid content was determined as described by Goossen et. al. (2018).  Dry weights of leaf blade and pseudostem fractions of pre-grazing samples were used to calculate DM yield, nutritional quality, and fatty acid contents of both fractions and total plant samples.

 There was an attempt to collect post-grazing samples on these same three days but there was too much trampling and some of the annual plants were literally pulled out of the soil making our method too variable and unreliable.  Therefore, we were unable to estimate dry matter intake and all our analysis is based on pre-grazing data.

Study 2. Pearl Millet Quality and Maturity -  In June of 2016, the Choiniere’s planted strips of several summer annual mixtures.  To minimize variability, we only sampled from a monoculture of pearl millet strip.  Because of a dry summer, we knew there would not be time for significant regrowth of the millet; therefore, we sampled more frequently through the grazing period of the one growth period.   There were six sample dates over a 19-day period beginning on July 18, the first day of grazing, when the stand had reached about 20 inches in height until August 5, when the forage was in early head stage.   Other than cutting to a stubble height of 15 inches, our sampling procedure and processing was the same as that described above with winter rye.

Study 3. Diurnal Variation in Water Soluble Carbohydrates of Pearl Millet Forage - The objective of this study was to evaluate hourly changes in water-soluble carbohydrates of pearl millet when it reached

maturities suited for grazing (18 to 20 inches in height) and harvesting for silage (25 to 30 inches).  On July 19 and July 26 of 2017, we collected two sets of samples at approximately one hour intervals starting at 7am and ending at 7pm.    Samples were cut with electric hand clippers from a 20 x 20 inch quadrat leaving a 4-inch stubble.  For each quadrat, the total number of tillers were counted and lamina (leaf blade) was separated from pseudostem (grasses at this stage do not have true stems but rather a combination of leaf sheaths, elongating culm, and pre-emerged new leaves all collectively called pseudostem).  Samples were microwaved for one minute and then placed in a forced air dryer for 48 hours until dry.  They were then ground and evaluated for WSC, crude protein (CP), acid detergent fiber (ADF), neutral detergent fiber (NDF),  and NDF digestibility with 48 hour fermentation time (NDFd48) using near infrared reflectance spectrophotometry (NIRS).