Balancing Dairy Heifer Rations
If you're planning to accelerate your heifer-rearing program you may want to examine your heifers' rations
Dairy farmers and their advisers have been paying more attention to raising heifers in the last five years, with good results. There are many reasons for this, including a new and better focus on raising milk-fed calves, greater awareness of how expensive it is to raise a heifer to calving age-about $2,000-new calf barns being built to provide better environments, and setting achievable goals.
There's industry-wide evidence farmers are doing a better job, too. CanWest DHI data showed Ontario has reduced the average age at first calving from 27.2 months in 2010 to 26.6 in 2013. This may not sound like a lot, but it's heading in the right direction, consistently, for the first time in several years.
Ontario is not alone because all provinces have reduced their average age at first calving over the same time period, according to CanWest DHI data. Examining cows' lifetime milk production records shows highest lifetime production is achieved from cows calving between 23 to 25 months of age. While there is still work to be done to move that average down, many producers are already meeting this goal.
Improve your heifers' growth
Better nutritional management can improve heifer growth. In 2011, the Calf-ETERIA project conducted a detailed survey of heifer-raising practices and only 38.5 per cent of Ontario farmers of 1,000 surveyed indicated they regularly balance their heifers' rations during the year. Having a properly balanced ration is important for improving heifer growth.
About 40 years ago, the nutritional advice for heifers was not to get them fat by feeding too much energy, because there was a risk of depositing fat in the mammary glands, which would reduce future milk yield. However, energy content in a heifer's diet may not be the only problem. Recent research from a joint project conducted by Virginia Polytechnic Institute and Brazilian researchers showed the impact of different protein to energy ratios on mammary development.
Their project studied five metabolized protein (MP) to metabolized energy (ME) ratios in pre-puberty and post-puberty heifers. Mammary development changes before and after puberty can have a strong effect on later milk production. The diets were formulated so they all met the average daily gain target of one kilogram per day. In the four-month study, 25 heifers were used, five per diet, with an average starting weight of 213 kg and an age of 7.8 months. Before the start of the experiment, all heifers were fed the same total mixed ration since weaning and gained about 0.7 kg per day.
In the experiment, the diets' protein content was increased by replacing corn with soybean meal. The protein to energy ratios were 33, 38, 43, 48, and 53 grams per mega calorie for the five treatments as shown in the table. The energy level in the five ratios was kept constant.
Ultrasound mammary gland measurements were done every 28 days to assess mammary fat deposits compared with milk-producing tissue changes from the diets. Ultrasound images were used to determine changes in mammary tissues based on the fact fat tissue reflects the ultrasound's sound waves by showing a whiter shade compared with the grey or darker pixels reflected back by other mammary tissue. The researchers carefully conducted pixel by pixel analysis of selected portions of the images.
Results of the five diets
The two highest dietary protein treatments provided more crude protein to energy than is recommended in the National Research Council's publication (2001) on dairy cattle's nutrient requirements. However, some researchers have recently suggested those recommendations may not be meeting the needs of heifers weighing more than 200 kg, which is why the highest levels were included.
There were no differences in the responses measured from the pre-puberty or post-puberty heifers. This indicated the responses to the diets were the same regardless of the heifers' maturity. The groups of heifers consuming the five diets had the same dry matter intake, dry matter digestibility and gained nearly one kilogram per day. Dietary crude protein content increased when soybean meal replaced some of the corn, and non-fibre carbohydrate decreased.
There was more nitrogen excreted in the heifers' urine when fed the higher levels of protein to energy in the diet. The main purpose of the project was to study the differences in mammary development, not the impact to the environment. The two lowest protein-to-energy diets, as shown in diets 33 and 38 in the table, showed significantly higher scores for brightness on the ultrasound images in the mammary glands, particularly in the mammary duct areas. The researchers concluded the diets with the lowest protein-to-energy ratios were likely to lead to a higher degree of fat deposits in the mammary glands when feeding for an average daily gain of one kilogram per day in heifers.
Unfortunately, the study did not record milk production, although the number of heifers in the study was best suited to examine mammary development differences, not variations in milk production.
Birth, puberty, pregnancy and lactation initiation are the key steps in mammary development, particularly when hormonal changes occur. In this study, the researchers took blood samples and reported a significant linear increase in the hormone IGF-1 as dietary protein increased. Along with other functions in the body, IGF-1 stimulates cellular proliferation in the mammary gland. Previous research has found a strong correlation between the concentration of IGF-1 in blood and the mammary gland.
When the fat tissue in the mammary gland grows in an undesirable way, as shown when the protein and energy contents of the diets are unbalanced, it can negatively impact milk-producing tissue development. The lowest dietary protein-to-energy treatment diet was not recommended for heifers when feeding for a high rate of gain.
From a practical standpoint, the study showed it is important to balance rations for growing pre-pubertal and post-pubertal heifers, when targeting higher rates of gain. Depositing fat in the developing mammary gland is affected by the diet's energy content and the ratio of metabolized protein to metabolized energy. In this study, the energy intake was constant in all the diets, so mammary fat development variations were related to the protein-to-energy ratio.
Consult your feed adviser on balancing your heifer rations regularly to achieve your goals if you're planning to accelerate your heifer-rearing program.
This article originally appeared in the May 2015 edition of the Milk Producer magazine.
Table 1. Composition of experimental diets with different metabolizable protein(MP): metabolizable energy (ME) and intakes (kilograms/day) of dry matter, crude protein and metabolizable energy.
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