Hypocalcemia, also known as milk fever, is a well-known transition cow disorder that occurs when a cow has difficulty maintaining calcium homeostasis after calving. The prevention, management, and treatment of this disorder has been highly researched due to hypocalcemia being a predisposing factor for other common transition cow diseases as well as impaired reproduction and productivity. This research has led to the clinical form of this disorder no longer being a major concern, rather the recent focus has shifted to subclinical hypocalcemia (SCH). Subclinical hypocalcemia is estimated to be experienced by approximately 50% of cows in their second lactation or greater and it is difficult to diagnose under field conditions.
In an effort to reduce the incidences of SCH, our industry has been reviewing the strategies we use to prevent milk fever. The prevention of milk fever is mostly done through nutrition by feeding a close-up diet that is low in calcium, has a negative dietary cation-anion difference (DCAD), or by the use of a calcium binder. All of these nutritional strategies work by creating a low blood calcium environment before calving, thereby increasing calcium mobilization from the bone. This essentially allows the body to pre-adapt to hypocalcemia and helps cows after calving because their body is already used to mobilizing calcium from the bone to meet demands.
A study published in the Journal of Dairy Science reported that feeding a restricted phosphorus diet during the close-up period also stimulated calcium mobilization from the bone. Walter Grunberg’s research group conducted a study in 2020 where cows were either fed a diet with adequate phosphorus content (AP; formulated for 0.30% P in DM) or low phosphorus content (LP; formulated for 0.15% P in DM) for at least four weeks before calving. During the close-up period, daily feed intake was restricted to 25 lb of DM per day (normal close-up diets are formulated for 27-28 lb of DM). Feed restriction was used to ensure that cows on the LP diet did not consume more than 20 g of phosphorus per day. Following calving, cows were fed the same lactating cow diet with adequate phosphorus content ad-libitum. After calving, blood samples were taken throughout the first 7 days in milk (DIM).
In this study, feeding the restricted phosphorus diet pre-partum successfully increased blood calcium compared to cows on the adequate phosphorus diet, and it also reduced the severity of clinical and SCH. Cows on the low phosphorus diet had numerically lower incidences of clinical and SCH compared to cows on the adequate phosphorus diet (1/15 cows with clinical and 8/15 cows with SCH vs 3/15 with clinical and 10/15 cows with SCH, respectively). Of the cows that did experience SCH, the cows on the low phosphorus diet only experienced low blood calcium for 2 days at most, while cows who were fed adequate phosphorus experienced low blood calcium for at least 3 consecutive days. There were significant treatment and time effects with a treatment by time interaction on blood calcium values (P ≤ 0.009). On days +0, +2, and +4 after calving, cows on the low phosphorus diet had significantly higher blood calcium values. Plasma CrossLaps, a marker for bone reabsorption, was also measured in the blood. The researchers found that there was a significant treatment, time, and treatment by time interaction for plasma CrossLaps (P ≤ 0.05) where cows who were on the low phosphorus diet had higher values on days +1, +2, and +7 after calving. This means that cows on the low phosphorus diet were mobilizing more bone after calving, which makes sense considering the results seen in blood calcium and incidences of disease.
So, should we all start feeding our cows a low phosphorous diet during the close-up period? Achieving such a low level of phosphorus to see these results can be difficult to do in a diet without some kind of assistance from a product. Hypophosphatemia, which is common during the first week of lactation and is when blood phosphorus values decrease, is a concern with feeding a low phosphorus diet. The dip in plasma phosphorus at calving was not mitigated in either treatment, and this can be concerning as it is believed to be associated with negative effects on health and reproduction of high producing dairy cows. This current study only followed cows until they reached 7 DIM, therefore, there may still be possible effects of a low phosphorus diet that could be seen later in lactation. With all that being said, cows fed a low phosphorus diet during the close-up period mobilized more bone and had higher blood calcium values after calving. More research is needed to determine longer-term effects of this diet, as well as how milk production and other parameters might be affected, and to compare such a diet to our current strategies.
— Emily Bourdeau