Grass lead tillers drop below the levels of crude protein requirements of lactating cows during late July.
Grasslands managed with traditional practices provide forage that is low in crude protein from early August to the end of the grazing season. This causes cow milk production to greatly decrease and calf weight gains to decrease to rates below their genetic potentials. The common remedy for this annual problem is to provide costly lick tubs for the cows and creep feed for the calves.
There is a biological solution built into grasslands. Perennial grasses reproduce vegetatively by activated growth of axillary buds. The axillary buds are inhibited from growing by the hormone, auxin, that is produced in the apical meristem and young developing leaves of second year lead tillers.
Partial defoliation by grazing graminivores that removes 25%-33% of the young leaf material from grass lead tillers at phenological growth between the three and a half new leaf stage and the flower stage during 1 June to 15 July also removes enough auxin that permits the growth hormone, cytokinin, to stimulate vegetative growth of secondary tillers from the meristematic tissue in axillary buds which will provide adequate crude protein for lactating cows to maintain milk production and calf rate of weight gain at near genetic potential from late July until mid October.
However, the growth and development of the stimulated secondary tillers requires mineral nitrogen to be available at 100 lbs/ac or greater. Intact grasslands contain 5 to 6 tons of organic nitrogen per acre which must be transformed by soil microbes into mineral nitrogen. Traditional grazing management practices usually only have enough soil microbe biomass to transform 50 to 60 lbs/ac of mineral nitrogen.
Lead tillers at vegetative growth stages between the three and a half new leaf stage and the flower stage produce surplus carbohydrates that can be exudated through the roots into the rhizosphere and increase the microbe biomass with partial defoliation by grazing graminivores that removed 25%-33% of the lead tiller leaf material.
The response from the rhizosphere microbes is not instantaneous. During the first two growing seasons, the rhizosphere change is no different than that on the nongrazed control. During the third year to sixth year the rhizosphere biomass increases. When the microbe biomass reaches a little more than 214 kg/m3 (955 tons/ac), the microbes will be able to mineralize around 112 kg/ha (100 lbs/ac) of mineral nitrogen.
When 100 lbs/ac of mineral nitrogen is available, the internal grass growth mechanisms of compensatory physiological growth, vegetative reproduction by tillering, nutrient resource uptake, and water use efficiency mechanisms can be fully activated. The twice-over rotation system is the only management strategy known to be able to maintain a large biomass of rhizosphere microbes that can mineralize greater than 100 lbs/ac of mineral nitrogen and to activate the four internal grass growth mechanisms.
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