Basis for Cows’ Dietary PreferencesRecently I visited Connie, a Mohawk Valley dairy farmer. I’ve been advising her in both her cow and crop management programs for many years. Her small herd of grass-fed organic Holsteins had once again begun to seriously chow down her complete mineral mix, dicalcium phosphate and salt. She told me that presently there’s less to her pastures than meets the eyes. After rather meager grazing in July (due to sparse rainfall), occasional rains blessed their farm throughout the rest of summer and early autumn. This enabled her herd to meet organic standards for grass-fed dairies.

Rains were sporadic through the rest of summer. Nonetheless, when Connie moved the milking herd from one paddock to another, it seemed that precipitation would arrive in time to recharge the paddocks during their rest and recovery phases. National organic grass-fed standards require that dairy animals in such programs consume an average of 60% of their daily dry matter intake in the form of mouth-harvested roughage. According to standards, this level of grazed forage consumption must be met, on average, over at least 150 days per year. The total grazing season must be at least 180 days (weather conditions permitting).

When pasture scores well in both quality and quantity, grazing cattle are able to meet most of their nutritional needs in the mouth-harvested form. This means that grazed forage is doing a good job of getting nutrients from the soil into the cows. Flush pastures, throughout most of May, provide so much nutrition to lactating dairy cattle that they often back way off free choice minerals – and even salt. Sometimes non-grass-fed cattle aren’t even interested in their grain.

A possibly serious nutritional train wreck that dairy graziers must be on the lookout for is grass tetany. This is most common during peak pasture flush in spring, but graziers should keep mentally attuned to the possibility of this frequently fatal metabolic disorder rearing its ugly head at any point during the grazing season.

Grass tetany may occur if cows are fed grass hay that’s low in magnesium, or whenever the mainstay of diet is cereal green-feed or silage – especially if potassium (K) in those feeds is high. High rumen levels of K often interfere with absorption of both calcium and magnesium. Fast-growing spring pastures are usually high in potassium (K+) and nitrogen (N+) and low in magnesium (Mg++) and sodium (Na+) ions. Affected cattle often have low blood calcium and low magnesium at the same time. Fall calving cows may also experience grass tetany during winter months.

Magnesium oxide (mag ox) may be routinely used as a buffer in these grain mixes for dairy cows. Producers should check with their nutritionist to make sure adequate amounts and proper sources are being used to prevent grass tetany.

Besides mag ox, another source of Mg is magnesium sulfate (Epsom salt), which is more palatable than mag ox. Mag ox is 53% – 55% Mg, while Epsom salt is only about 10% Mg. Mag ox, when unit-priced for elemental Mg, costs about one-sixth as much as Epsom salt. Convincing cows to consume less palatable mag ox may be effort well-spent economically.

A lot of livestock people offer their cattle a blend of the two Mg sources. For beef and dairy cattle people with total mixed ration feeding systems, blending in minerals, particularly Mg sources, is a no-brainer – palatability problem solved. But a plus for the more expensive Epsom salt is that its sulfate (when dissociated from its Mg) is an anion (negatively charged particle) bearing two extra electrons. The trait helps keep anions and cations (positively charged particles) in balance in the rumen. Maintaining this balance helps cattle avoid milk fevers. Because of that factor, if I have to choose one Mg source for dry cows, it will usually be Epsom salt.

Cattle – and all ruminants, for that matter – have senses of taste and smell far more sensitive than those of humans. The darker green spots in a well-grazed pasture mark the deposits of what fell from the north ends of south-bound cows. The vegetation is taller there, remaining ungrazed, because the animals smell that such forage contains too much nitrate, compliments of the brown splatter. Probably the actual threat of nitrate poisoning is minimal. But the material smells bad to cows, so cattle aren’t interested in learning what it tastes like.

The first situation I ever encountered in which cattle rejected forage was in the early 1960s. My family lived in Prattsville in Greene Co., on a small farm with beeves and horses. At the edge of our township, straddling the Delaware Co. line, was a slaughter facility which processed large numbers of cattle, culled from a then-thriving dairy industry. Slaughter waste products (mostly inedible intestines and blood) were ground up, mixed with water and spread on a flat meadow situated between Route 23 and Schoharie Creek. This fertilizer concoction grew hay like crazy, which was baled, then offered to cattle on nearby farms. Cattle would almost starve before they considered eating such tainted hay.

Here’s one more example of ruminant taste preferences: In September 2008, my son Will, who worked for the World Wildlife Fund at the time, attended a conference in Central Oregon. He paid for my airfare to visit for a few days after the conference. We traveled through that state’s high plains. Agriculture there was basically beef cattle and deep-well irrigated hay. Altitude of 5,000 – 6,000 feet pretty well ruled out corn. I was fascinated, watching pronghorn antelopes grazing just outside the green disks caused by pivot irrigation. I’d heard that sometimes deep wells hit major mineral deposits that must be filtered out prior to human consumption. Alfalfa didn’t need filtered water, but the antelopes preferred unirrigated browse that caught whatever rain fell in this Cascade Mountain rain shadow.