Lactating cow diets containing phosphorus (P) concentrations of 0.35 to 0.36% (dry matter basis) are recommended by the national committee of dairy nutritionists in 2001. This is a lower P concentration than what was recommended in the previous dairy feeding guidelines of 1989. Guidelines for feeding P in 1989 were largely based upon studies published prior to 1950 that used cattle grazing pastures deficient in P and likely other nutrients as well. These grazing cattle had decreased calf crops.
New research indicates that the 2001 recommendations are just fine. Wisconsin researchers have conducted several experiments in which they fed different dietary concentrations of P and measured both milk production and reproduction. For 308 days of lactation, Holstein cows were fed a diet containing 0.31, 0.40, or 0.49% P by increasing the amount of monosodium phosphate in the diet (Wu et al., 2000). Overall milk production was not different among the three groups (24,361 lbs average) although cows fed the 0.31% P diet produced less milk during the last third of lactation.
The number of days to first estrus and to the first AI was the same for cows fed the 0.31% and the 0.49% P diets but were greater for cows fed the 0.40% P diet. Unexplainably, the number of services per conception by 206 days in milk increased as the intake of P increased.
A second experiment was done that lasted for two consecutive lactation cycles. Holstein cows were fed one of two diets. The low P diet contained 0.31 to 0.38% P and the high P diet contained 0.44 to 0.48% P throughout the 2 years (Wu and Satter, 2000). During the first lactation cycle, cows performed the same regardless of dietary P concentration (19,831 lbs average in 308 days).
All cows were pregnant by 230 days in milk. During the second lactation cycle, milk production was again the same between the two groups of cows (21,784 lb average in 308 days). In both years, the cows fed the higher P diets did not have better pregnancy rates nor did they have fewer days open at 230 days in milk.
In a third Wisconsin study published in 2004 involving a lot more cows (267), diets of 0.37 and 0.57% P supported similar amounts of milk production and similar conception rates (Lopez et al., 2004).
In summary, these studies indicate that productive and reproductive performance will not be improved by increasing the dietary concentration of P above 0.37 to 0.38% (DM basis). It was only when diets got down to 0.31% for two years that high producing cows showed potential harmful effects of low P intake (low P in sampled rib bones). Even if P intake is somewhat deficient during the early days postpartum when DMI is low, cows are likely able to mobilize P (1.3 to 2.2 lb) from bone to meet a temporary P deficiency and then to replace the bone P when P intake exceeds P requirement later in lactation.
By Charlie Staples
Dairy Update Quarterly Newsletter
University of Florida IFAS - Department of Animal Sciences