This month we will continue our protein nutrition series with the classes of horses having the most demanding protein needs: the broodmares. It would make sense that this group of horses is the most sensitive to changes in protein nutrition, as they are continually supplying nutrients for the growing fetus or foal. Shortchange your mare and you may be short changing your future generation. But let’s take a look at what these animals need, so that we can avoid any potential pitfalls in our feeding strategies.

Before we start feeding a mare for gestation, we at least need to get her pregnant first. Therefore proper nutrition of the broodmare does start before conception. As long as the mare is fed adequate amounts of protein (no higher than required by a maintenance horse) there should be no nutritionally related reproductive issues. However, mares which are deficient in protein are slower to begin cycling normally and have a higher rate of early loss of pregnancy. Short changing the mare on the feed bill will definitely not save money in the long run. Always begin the breeding season with a proper feeding program for optimal results.

 After the mare has conceived, her early pregnancy requirements are not much different from when she was open. Just like with her energy needs, her changes in protein requirements are really quite minimal at the beginning of her pregnancy. As the rate of growth of her developing fetus increases, she must have a greater supply of amino acids in her diet. Looking at Table 1, you can see that the greatest increase in her protein requirements occurs in the last two months of pregnancy. Table 2 places these daily requirements into a simpler expression of the total percent protein that your mare will need depending on the amount of feed she is consuming.  Just as you can imagine, the more the mare consumes, the lower the total percentage of crude protein needed in the diet.  In fact, this is usually what happens. Most mares will voluntarily consume more feed as her energy needs go up, thus also meeting her protein requirements.

It might be surprising to look at the values in Table 2 compared to the typical amount of protein present in a commercial feeds.  Most feeds designed for broodmares range between 14 and 16% crude protein, yet the overall protein percentages needed in the diet are much lower. So why do feed manufacturer’s offer such high levels of protein in their products?   For one, most of the feeds are really designed to meet the needs of the lactating mare, rather than the gestating mare. Additionally, you may remember from earlier articles that forage protein is not digested as fully as the protein which comes from concentrates. While protein requirements do try to account for some of this variance, an average digestibility value combining both forages and concentrates is used to calculate the total amount needed in the diet. Furthermore, the guidelines for protein intake are based largely from research with horses consuming mixed diets usually offering an equal proportion of concentrates and forages. Thus if more of your horse’s diet is forage, you must consider that when selecting an appropriate concentrate.   Remember as well, that legumes are digested to a further extent than grass hays, thus also offering not only more protein on a concentration basis, but also being absorbed more thoroughly by the horse. A general guideline would be, that if feeding high quality legume hays, the amount of protein required in the concentrate could be much lower (as low as 10-12% CP). If feeding primarily grass hays, then you should select a concentrate closer to 14% CP.

Wt of horse (lb)	Early gestation	5 mo.	6 mo	7 mo	8 mo	9 mo.	10 mo.	11 mo.
900	507	551	566	586	611	641	676	718
1000	562	611	628	650	677	711	750	796
1100	619	673	691	715	746	782	826	877
1200	675	754	755	781	814	854	902	957
1300	731	794	817	845	881	924	976	1036

Table 1. Protein requirements (g/d) for a gestating mare based off her initial non-pregnant weight.

Table 2. The total percent crude protein needed in the diet for a pregnant mare throughout gestation. These values are based from the total intake on a dry matter basis. However, percent total protein is represented on an as fed basis, as is represented on feed tags.

The lactating mare will consume more feed than the typical maintenance horse as her energy demands have increased greatly.   Just like with energy, it is the lactating mares who really have the biggest nutritional demands of any of our horses. Compare the grams of protein needed per day in Table 3 to Table 1. You can see that her protein needs have more than doubled. If mares are deficient in protein, they will be unable to produce as much milk as those at an adequate plane of nutrition, and may end up losing weight. This is certainly undesirable, as this is also the time period when most mares are being rebred as well.   Mares do a fairly good job of producing milk though certainly not equivalent to a Holstein cow! Mare’s milk production usually averages about 3% of her body weight, with that value tapering off to about 1.9% of her body weight during late lactation. The protein concentration in the milk is the highest during the first 22 days of lactation (when foal growth is very rapid) and thereafter plateaus throughout the rest of her lactation.

Wt of horse (lb)	Early lactation	3mo.	4 mo	5 mo
900	1234	1180	1124	1069
1000	1369	1309	1247	1186
1100	1507	1441	1372	1306
1200	1646	1573	1498	1426
1300	1781	1702	1621	1543

Table 3. Crude protein requirements in grams per day for the lactating mare.