Phosphorus Fertilization of Tall Fescue Pastures May Protect Beef Cows from Hypomagnesaemia and Improve Gain of Nursing Calves

T. Ryan Lock, Graduate Research Assistant, Robert L. Kallenbach, Assistant Professor, Dale G. Blevins, Professor, Timothy M. Reinbott, Research Associate, and Greg J. Bishop-Hurley, Post-Doctoral Fellow, Department of Agronomy, University of Missouri, Columbia, 65201; Richard J. Crawford, Jr., Research Assistant Professor, and Matt D. Massie, Research Specialist, Southwest Research and Education Center, Mount Vernon, MO, 65712; Jeffrey W. Tyler, Professor, Department of Veterinary Medicine and Surgery, University of Missouri, Columbia, 65201

Abstract

Previous research has linked soil phosphorus level with forage magnesium concentration in tall fescue (Festuca arundinacea Schreb). Understanding this relationship might reduce the incidence of sub-acute hypomagnesaemia and/or grass tetany in cattle (Bos taurus). Our objective was to determine if P fertilization of tall fescue pastures affects the mineral element status of lactating, mature cows in spring and performance of their nursing calves. The treatments were: (i) tall fescue pasture fertilized to achieve 30 lb/acre of Bray I P; (ii) tall fescue pasture not fertilized with P but animals were given access to a mineral supplement that contained 12% Mg; and (iii) tall fescue pasture not fertilized with P and no supplement provided. With two exceptions, cows in all treatments showed similar serum and cerebrospinal fluid Mg concentrations. Average daily gain of nursing calves was 0.2 lb/day higher in the P-fertilized treatment compared to the other treatments. Thus, improving soil P level may alleviate symptoms of sub-acute hypomagnesaemia in cows and, more importantly, increase calf gain.

Introduction

Grass tetany is a nutritional disease that affects lactating cattle fed Mg-deficient diets. In the lower Midwest, grass tetany outbreaks are most common from mid-February through April (6). During the early part of the grass tetany period, cows are often fed mature cool-season grass hay and/or stockpiled tall fescue with a low concentration of Mg (Fig. 1). Grass tetany is most prevalent when cows are moved from these dormant stockpiled forages or hay to lush spring growth (6). To further complicate matters, approximately 60% of cows in the region give birth during February and March, and parturition increases cow demand for Mg by nearly three-fold (11). These factors combine to make late winter and early spring a difficult time to avoid grass tetany.

Fig. 1. A beef cow grazing stockpiled tall fescue in February. This forage often contains a low concentration of Mg and puts cows at risk for hypomagnesaemia.

Often, cows with grass tetany have a serum Mg concentration below the lower threshold of 18 ppm (5). Grass tetany is potentially fatal, but it is diagnosed in less than 1% of the mature female beef cow population each year (3). Perhaps more important is the precursor to grass tetany known as sub-acute hypomagnesaemia. Over 40% of beef cows (n = 2261) in one Irish survey during the early spring grazing season had sub-optimal (< 19 ppm) serum Mg (9). Despite this, the cows did not display classic symptoms of grass tetany. While not displaying severe symptoms of disease, 20% reductions in milk production and feed intake have been reported for cows (6,14). Thus, sub-acute hypomagnesaemia may have greater economic importance than grass tetany simply because of the larger numbers of cows affected.

Recent Missouri research showed that raising soil P levels in the plow layer from below 16 to at least 25 lb/acre (Bray I P) increased the concentration of Mg in tall fescue during early-spring when compared to unfertilized pastures (8,12). Thus, cattle may consume their Mg requirement while grazing, eliminating the need for Mg supplementation. Our objective was to determine if P fertilization of tall fescue pastures affects the mineral element status of lactating, mature cows in spring and performance of their nursing calves.

Field Scale Grazing Experiment

A detailed description of the experimental site, as well as fertilization and cultural practices, has been published previously (8). In brief, the study was conducted on established ‘Kentucky 31’ tall fescue pastures with an endophyte infection level of 67% located at the Southwest Missouri Research and Education Center near Mt. Vernon, MO (37°04’N, 93°53’W; elevation 1150 ft). The soil series was a Creldon silty clay loam (fine, mixed, mesic Mollic Fragiudalf). These pastures are typical of the region and had 6 lb of Bray I soil P per acre in the top 6 inches of soil before the study started.

Treatments. The three treatments were: (i) tall fescue pasture fertilized to achieve Bray I soil P at 30 lb/acre; (ii) tall fescue pasture not fertilized with P but animals were given access to a mineral block supplement that contained 12% Mg; and (iii) tall fescue pasture not fertilized with P and no Mg supplement was provided. For simplicity, these treatments will be referred to as P-fertilized, supplement, and control, respectively. In the supplement and control pastures Bray I soil P was 6 lb/acre.

Animals. Twenty-seven Angus and Angus crossbred cow/calf pairs were used. Cows received tall fescue hay and stockpiled tall fescue pasture during a 4-wk acclimation period. No Mg supplement was given to the cows during the acclimation period. Cows ranged in age from 6 to 12 years. The average calving dates for the herd were 23 February 2000 and 13 February 2001. In 2000, all cows but one had nursing calves at their side during the experiment and in 2001 all cows had nursing calves. Two weeks before grazing began each year, blood samples were collected by jugular puncture from each cow and the Mg concentration in the serum was determined. Based on these preliminary Mg concentrations in serum, cows were stratified to low, medium, or high groups. Equal numbers of low, medium, and high animals were placed in each of nine groups and then assigned at random to one of nine 2-acre pastures (Fig. 2).

Fig. 2. Three cows with calves were arranged in each of nine 2-acre pastures.

We offered cattle in the P-fertilized and control treatments standard salt blocks containing NaCl only. The Mg mineral blocks offered to cattle in the supplement group also contained the following: crude protein, 2%; crude fiber, 5%; NaCl, 17%; Ca, 6%; P, 1%; and vitamin D₃, 15000 i.u./lb.

Grazing management. We conducted the experiment during the typical grass tetany season, which begins approximately four weeks before new growth starts in spring and ends approximately six weeks later (6). Cows grazed from 15 February to 11 April 2000, and 6 March to 1 May 2001. The animals strip grazed stockpiled tall fescue each year until spring growth began, except when weather conditions prevented its use for a short period in 2001. Forage mass was measured at the beginning of the experiment and at 14-day intervals thereafter. Forage was allocated every 7 days to improve utilization and temporary electric fences restricted the animals to the allocated area. Average forage dry matter availability from each “point-in-time” measurement was 5.0% of cow body weight. Cattle were continuously stocked on pastures after they consumed all of the stockpiled reserves or when spring growth began. In 2001, cold weather in March delayed grass growth. Thus, cattle were restricted to 0.5 acre in each pasture from 29 March until 17 April. During this time, cows were provided tall fescue hay while pastures grew. The hay contained the following nutritive value: crude protein, 9.6%; acid detergent fiber, 48.2%; neutral detergent fiber, 76.7%; Mg, 0.21%; Ca, 0.51%; K, 1.38%; and P, 0.15%.

Animal measurements. Cows and calves were weighed when grazing started and at 14-day intervals thereafter. At these times we collected blood samples from cows by jugular puncture. There were five sampling dates in 2000 and again in 2001. In addition, veterinarians collected cerebrospinal fluid at the beginning of the trial and at 28-day intervals thereafter for a total of three samplings each year. Prior to cerebrospinal fluid collection, cows received a local injection of 0.002 oz. of Lidocaine. Cerebrospinal fluid was collected from the lumbosacral cistern with 18-gauge, 6.0-inch-long styletted needles.

Blood samples clotted overnight in a refrigerator. Serum was harvested and stored at -4°F until analyzed for mineral elements and 25-hydroxyvitamin D concentration (vitamin D). The cerebrospinal fluid samples were stored at -4°F until analyzed for mineral element concentration.

Mineral element and vitamin analyses. Serum and cerebrospinal fluid were analyzed for K by flame photometry, Mg and Ca by atomic absorption, and P colorimetrically (10). All mineral analyses were conducted in duplicate. In addition to mineral element analyses, serum was analyzed for vitamin D using a commercially available radioimmuno assay kit developed by DiaSorin, Inc., Stillwater, MN (15).

Statistical analysis. We used a split-split plot in time model with repeated measures to analyze the data (4). Main plots were three pasture treatments and split plots were three (cerebrospinal fluid) or five (serum) harvest dates, and split-split plots were years. Main plots were replicated three times, with three cows in each main plot. Means for all data were separated using Fisher’s protected least significant difference. The 0.10 alpha level was used to test our hypotheses. For each variable except serum Mg, data were pooled across 2 years because error variance was homogeneous. Responses to treatments differed between years for serum Mg and analyses were conducted for each year separately.

Mineral Element and Vitamin Concentrations in the Serum of Cows

Magnesium. In 2000, serum Mg concentration of cows ranged between 15 and 20 ppm (Table 1). At times, cows in all treatments had Mg concentrations that were less than the lower threshold of 18 ppm, although no outward signs of grass tetany were noticed. No treatment differences existed in serum Mg concentration of cows until the last sampling, 56 days into the experiment. At this sampling, cows in the P-fertilized and supplement treatments had serum Mg levels that were 20% greater than those from the cows in the control treatment. This increase in serum Mg for P-fertilized and supplement-treated cows loosely mirrored dietary Mg (8).

Table 1. Magnesium concentration (ppm) in the serum of cows for 56 days in 2000 and 2001. Day zero corresponds to 15 February 2000 and 6 March 2001. Cows grazed tall fescue grown on a soil with Bray I P at 30 lb/acre (P-fertilized), tall fescue grown on a soil with Bray I P at 6 lb/acre while supplemented with Mg free choice (Supplement), or tall fescue grown on a soil with Bray I P at 6 lb/acre but no Mg supplementation (Control).

Treatment	Days from start of grazing					LSD (0.10)
Treatment	0	14	28	42	56	LSD (0.10)
2000
P-fertilized	19	18	18	17	20	1.4
Supplement	19	19	17	17	19	1.4
Control	19	18	16	15	15	1.4
LSD (0.10)	NS	NS	NS	NS	2.2	--
2001
P-fertilized	17	11	18	19	19	1.8
Supplement	16	16	19	18	18	1.8
Control	17	14	18	18	18	1.8
LSD (0.10)	NS	1.8	NS	NS	NS	--

In 2001, the treatments responded differently than in 2000. The concentration of Mg in the serum of cows ranged between 11 and 19 ppm (Table 1). Treatment differences were only present on Day 14, when cows in the P-fertilized treatment had a serum Mg concentration that was 34% less than those in the supplement treatment and 23% less than the cows in the control treatment. At the next blood sampling (Day 28) all cows showed equal serum Mg concentrations and remained that way for the rest of the experiment.

The responses in 2001 are perhaps explained by the weather in the 4 days prior to the blood sampling, along with the milk production of the cows. Nearly 6 inches of snow fell at the site on Day 10. These weather conditions may have limited dry matter intake of all cows. Thus, cows relying on forage alone for Mg may not have eaten enough dry matter to receive their daily Mg requirement. As a result, serum Mg concentrations dropped 40% for the P-fertilized and 20% for the control treatments, respectively, compared to Day zero (Table 1). In contrast, cows in the supplement treatment did not depend on forage alone for Mg, and as a result their serum Mg level did not change during the first 14 days of experimentation. When the weather became milder (by Day 28), the Mg concentration in the serum of cows in the P-fertilized and control treatments returned to initial values and were equal to the supplement treatment.

Another possible reason cows in the P-fertilized treatment showed a lower serum Mg concentration than the cows in the supplement and control treatments may be due to greater milk production. Performance of the nursing calves in this experiment suggests that cows in the P-fertilized treatment may have produced more milk than cows in the supplement and control treatments (Fig. 3). Calf gain during the first 90 days after birth depends almost entirely on cow milk production (1). In almost all measures, forage nutritive value after spring growth began (after Day 28) was greater for the P-fertilized treatment than for the two low-soil-P treatments (7,8). This increase in forage nutritive value could conceivably drive greater milk production and calf gains.

Fig. 3. Average daily gain of nursing calves for the 56-day experiment. Data are means across two years. Cows grazed tall fescue grown on a soil with Bray I P at 30 lb/acre (P-fertilized), tall fescue grown on a soil with Bray I P at 6 lb/acre while offered Mg mineral blocks free choice (Supplement), or tall fescue grown on a soil with Bray I P at 6 lb/acre and no Mg supplement (Control). Different capital letters indicate treatment differences at the 0.10 alpha level.

Calcium, phosphorus and potassium. The concentrations of Ca, P, and K in serum were unaffected by treatments and were within normal ranges reported in other studies (2) (Fig. 4). Concentrations of Ca and K in serum did change slightly over time, typically mirroring changes of these elements in the forage (8). It seems likely that cows mobilized Ca from storage tissues to replace serum Ca lost due to lactation. It is somewhat surprising that no treatment differences existed for serum P concentrations even though forage P concentration of the P-fertilized treatment was 30 to 50% greater than the control and supplement treatments at each date (8). The change in serum K concentration of cows over time may be due to cows coping with increased dietary K (13) as their diet changed from stockpiled forage (or hay) to new spring growth (8).

Fig. 4. Calcium, phosphorus, and potassium concentrations in the serum of cows for the 56-day experiment. Data are means across three treatments and two years. Day zero corresponds to 15 February 2000 and 6 March 2001. Different capital letters for an element indicate differences between dates at the 0.10 alpha level.

Vitamin D. The relationship between vitamin D, Ca, and Mg in the serum is important to lactating cows at the onset of lactation. The production of vitamin D signals for the conservation of Ca and its utilization from bone tissue. However, low serum Mg concentrations can inhibit production of vitamin D (6).

Vitamin D in serum ranged between 23 and 38 ppb and was not affected by the treatments (Fig. 5). Sampling date did influence its concentration however. Over the 56 days, the levels increased nearly 40% when compared to the initial date. This increase in vitamin D concentration appeared to follow the lactation curve of cows. Apparently, the Mg status of cows in all treatments was never low enough to inhibit the production of vitamin D. Interestingly, the mineral supplement offered to cows in the supplement treatment did not have an effect on the concentration of vitamin D in their serum, even though it contained vitamin D at the rate of 15,000 IU/lb.

Fig. 5. 25-hydroxyvitamin D concentration in the serum of cows for the 56-day experiment. Data are means across three treatments and two years. Day zero corresponds to 15 February 2000 and 6 March 2001. Different capital letters indicate differences between dates at the 0.10 alpha level.

Mineral Element Concentrations in the Cerebrospinal Fluid of Cows

Cerebrospinal fluid is thought to be a better indicator of the mineral status of cows than serum, especially for disorders of the central nervous system (16). None of the cerebrospinal fluid element concentrations measured was affected by the treatments and all were similar to previously reported values for cattle (2,16). The concentration of Mg, P, and K did increase slightly over time for all treatments (Table 2). The concentration of Ca in the cerebrospinal fluid did not change over time and when pooled across treatments, years, and sampling dates was 52 ppm.

Table 2. Magnesium, P, and K concentrations (ppm) in the cerebrospinal
fluid of cows. Data are means across three treatments and two years.
Day zero corresponds to 15 February 2000 and 6 March 2001.

Cerebrospinal fluid element	Days from start of grazing			LSD (0.10)
Cerebrospinal fluid element	0	28	56	LSD (0.10)
Mg	18	19	20	0.5
P	14	15	16	0.8
K	154	159	163	3.0

Weight Gain of Calves

In both years, calves from the P-fertilized treatment gained 10% more live-weight per day than calves from the supplement and control treatments (Fig. 3). Calves were under 75 days of age and capable of little rumination (1), thus this response is thought to have occurred primarily due to greater milk production of cows in the P-fertilized treatment (not quantified). The greater milk production may have been associated with the higher herbage nutritive value of those pastures.

Conclusions

Phosphorus fertilization of tall fescue and Mg supplementation of cows resulted in similar serum Mg concentrations, however at times neither of these treatments gave higher Mg concentrations than the control. Also, cows from all three treatments had serum Mg concentrations that were at times below the grass tetany threshold of 18 ppm. Phosphorus fertilization of tall fescue is thought to have alleviated the decreased milk production often associated with the sub-acute hypomagnesaemia because in both years, nursing calves in the P-fertilized treatment gained 10% more live-weight per day than calves in the other treatments. Raising soil Bray I P to recommended levels might alleviate the subtle symptoms of hypomagnesaemia, but more importantly for cattlemen, it improves calf performance.

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